Shock and vibration testing for rolling stock equipment, explained for the people who scope it.

As a manufacturer of equipment for rail vehicles, sooner or later a customer will ask you to prove suitability. That request usually arrives as a single line in a specification: shock and vibration tested to BS EN 61373. What it does not state is which test category applies to your product, and that one decision changes almost everything. It sets how hard the equipment is shaken, how the fixture is built, how long the programme takes, and what it costs.

This article looks at the practical differences between Category 1 (body mounted) and Category 2 (bogie mounted) equipment, how to work out which one applies to your equipment, and the common classification mistakes that quietly cost suppliers time and money. If you already know your requirements and are looking for more information or a quotation, click here.

Where the requirement comes from

BS EN 61373 rarely turns up in isolation. Most suppliers have a requirement to meet it through EN 50155, the standard for electronic equipment on rolling stock, which calls up EN 61373 for the mechanical aspects of testing. EN 50155 is often stated as a requirement by the train builder or system integrator as part of contracts, which in turn sits under the wider type-approval framework that gets a vehicle accepted into service. In other words, the requirement often reaches you via the end customer, not directly from a regulator.

The useful detail for a supplier is the default. Unless your customer says otherwise, EN 50155 applies EN 61373 Category 1 Class B to body mounted electronics. That matters, because Class B is treated as the conservative body mounted level, and the standard states that it is to be used whenever the mounting position is not yet clear.

Category 2 is sometimes mistakenly described as the default, but that is a common misreading. Category 2 is only the fallback test level for vehicles with a single level of suspension, such as wagons and freight trucks. For normal passenger stock with two suspension stages, body mounted equipment is Category 1. Getting this correct at the beginning can save a round of re-testing later.

The three categories

EN 61373 sets test severity on one thing only: where the equipment sits on the vehicle. Not how delicate it is, not how important it is, just its location. The three categories work like this.

Category	Class	Where it is mounted
Category 1 Body mounted	Class A	Cubicles, equipment and components mounted directly on or under the car body.
	Class B	Anything mounted inside an equipment case which is itself mounted on or under the car body. This is the default when the location is not yet known.
Category 2 Bogie mounted	—	Cubicles, subassemblies, equipment and components mounted on the bogie of a railway vehicle.
Category 3 Axle mounted	—	Subassemblies, equipment and components mounted on the wheelset (axle) of a railway vehicle.

EN 61373 test categories for equipment installed on rail vehicles.

Category 1 vs Category 2: what actually changes

The gap between body and bogie is large, and it is worth seeing in real numbers. Take the vertical axis, which is always the most severe.

Vertical axis severity (the worst case)	Cat 1 Class A	Cat 1 Class B	Cat 2
Functional random vibration (m/s² RMS)	0.750	1.01	5.40
Simulated long-life vibration (m/s² RMS)	4.25	5.72	30.6
Shock pulse, half-sine peak	30 m/s²	30–50 m/s²	300 m/s² at 18 ms

Vertical axis test levels, from BS EN 61373:2010 Tables 1 to 3.

Moving from Category 1 Class B to Category 2 is roughly five times the vibration amplitude. Because vibration energy rises with the square of amplitude, that is closer to twenty five or thirty times the energy going into your product. Shock follows the same pattern. A body mounted item sees a half-sine pulse of 30 to 50 m/s². A bogie mounted item sees 300 m/s², around six to ten times harder. Category 3 at the axle is harsher again, with a 1000 m/s² pulse.

The reason for this difference is suspension. The car body sits above both the primary and secondary suspension, which dampen most of what the track throws at the vehicle, so body mounted equipment lives a relatively gentle life and is tested up to 150 Hz. The bogie frame sits above only the primary suspension, much closer to the wheel-rail interface, so it sees larger and faster inputs and is tested up to 250 Hz. Axle mounted equipment has no suspension between it and the rail at all, which is why Category 3 runs to 500 Hz. The closer you get to the wheel, the harsher the environment gets.

Class A or Class B: Which is right for you?

For a lot of equipment Category 1 is the most applicable category, so the choice between Class A and Class B is the one worth getting right. It is the part of the standard that manufacturers or suppliers are often unsure about, and the place where a small misread quietly becomes an under-test.

What actually separates the two? Both are body mounted, so they share the same frequency range, up to 150 Hz for equipment up to 500 kg, and they share the same shock test, 30 m/s² vertical and transverse and 50 m/s² longitudinal, at 30 ms, for both classes. The thing that does change between Class A and Class B is the random vibration amplitude.

Category 1 functional random vibration (m/s² RMS)	Vertical	Transverse	Longitudinal
Class A (mounted directly on the body)	0.750	0.370	0.500
Class B (inside a body-mounted case)	1.01	0.450	0.700

_{Category 1 functional random vibration levels, Class A against Class B. The shock test and frequency range are the same for both.}

Class A applies to equipment whose own enclosure bolts directly to the car body, a complete cubicle, or a component fixed straight onto or under the body. Class B applies to anything mounted inside a further case or cubicle that is itself body mounted, in effect a box within a box.
A control module, power supply or circuit board sitting inside a body-mounted cabinet is Class B. It is also the class the standard states you use whenever the final mounting position is not yet decided.

Class B is the more severe of the two. For functional random vibration the vertical level is 1.01 m/s² RMS against 0.750 for Class A, and for simulated long-life it is 5.72 against 4.25, around 35% higher in each case. Because the ASD level scales with the square of the RMS, that 35% in amplitude works out near 1.8 times the vibration energy density.

The reason for this is how the levels were derived. The standard sets the Class A functional level at the average measured service level plus one standard deviation, and the Class B level at the average plus two standard deviations. The wider bound for Class B reflects the extra uncertainty when equipment sits inside an enclosing structure, where the cabinet can respond to the input and pass on more than the bare body sees. It is a deliberately more cautious level for the less predictable case.

In practice the question comes down to what your equipment is and where it is designated for installation:

A standalone cubicle or enclosure that bolts to the car body is Class A.
A module, sub-assembly or component installed inside a body-mounted cubicle is Class B.
If the mounting position is not yet confirmed, Class B is the conservative default.

One point that sometimes catches people out is that if you build a complete cubicle and test it to Class A, you have qualified the cubicle as a body-mounted item, but you have not automatically qualified any modules inside it to Class B in their own right. Where a customer needs the internal equipment certified separately, that is a Class B test in its own right.

Because the only difference between the classes is amplitude, Class B fully contains Class A. If you are genuinely unsure, or you want one certificate that covers either mounting arrangement, testing to Class B is the safe choice and is exactly what the standard intends. If you are trying to decide which category best fits your equipment and installation, send us the details and we will confirm the right class prior to costings.

The classification trap

Because all of the categories are so far apart in terms of energy input, getting the category wrong is not a minor slip.

Under-classifying can be dangerous. If a bogie mounted unit is tested to Category 1, it will sail through a test that is a fraction of its real service environment. The first time it meets the true environment is in the field. That is how you end up with fatigue cracks, loosened fasteners and warranty claims on equipment that holds a valid-looking test report.

Over-classifying has no real upside. Defaulting everything to Category 2 or 3 to be safe means bigger shakers, more demanding fixtures, longer test time and a real risk of failing equipment against conditions it will never meet. You pay more, and you may fail a perfectly acceptable product.

The fix is simple. Confirm the intended mounting location (and orientation) with your customer in writing before you scope the test. If it genuinely is not decided yet for a body mounted electronic item, Category 1 Class B is the sensible default. If there is any doubt about whether something counts as body or bogie, then we would be happy to have a quick conversation to discuss, before you commit to any test plan.

What the test actually involves

Whichever category applies, the shape of the programme is the same. Testing is carried out in three axes, vertical, transverse and longitudinal, each with its own level, with vertical the highest. There are three parts to it.

Simulated long-life vibration. Higher levels with the equipment switched off, for a total of 15 hours, normally 5 hours in each axis, to represent years of service in a manageable time.
Shock. Eighteen half-sine pulses, three in each direction across the three axes, to simulate rare in-service transient events.
Functional random vibration. The equipment operates whilst it is shaken, long enough to exercise all of its functions, so you can show it works correctly and without issue in its real environment.

The higher the category, the harder the engineering. As the upper test frequency climbs from 150 Hz for body, to 250 Hz for bogie, to 500 Hz for axle, the fixture has to work harder. It must be stiff enough that its own resonances sit above the test frequency range, yet light enough not to overly raise the shaker's force requirements. That balance is straightforward at Category 1 and can become a genuine design exercise at Category 2 and 3. It is one of the reasons we like to review fixture drawings early, ensuring the fixture is suitable before the equipment ever reaches the shaker.

When the installed orientation isn't fixed

The three axes do not all carry the same level. Vertical is always the highest, and the two horizontal axes, transverse (across the train) and longitudinal (along it), are lower and differ from each other. The standard handles the obvious unknown case directly. If the installed orientation is unclear or unknown, you test all three axes at the vertical level. That is fully compliant and about as conservative as it gets.

A more common situation is narrower than that. The vertical axis is fixed by how the unit bolts down, but the equipment could be installed rotated by 90 degrees in the horizontal plane. You know it is body mounted, you just do not yet know which face will point along the train and which across it. Forcing the full vertical level onto both horizontal axes in that case is heavier than the equipment will ever need.

The sensible approach is to keep vertical at the vertical level, then test both horizontal axes to the higher of the transverse and longitudinal levels. Doing so means that whichever way the unit ends up being installed, each of its horizontal axes has already seen at least the level it will meet in service, so every rotation is covered without over-testing using the vertical direction levels.

One detail is worth knowing here. Which horizontal level is the higher of the two flips with category. For Category 1 body mounted equipment, the longitudinal level is actually the greater (for example, 0.700 m/s² longitudinal against 0.450 m/s² transverse for Class B functional vibration). For Categories 2 and 3, transverse is the greater. So test both to the higher level means the longitudinal level for body mounted gear and the transverse level for bogie and axle mounted gear.

This is sound engineering judgement and is widely accepted, but it interprets the standard rather than following the clause word for word, so treat it as a deviation to be managed, not a default to apply quietly. State the chosen orientation and excitation levels in the test specification, record them in the report, and agree the approach with your customer. The standard expects exactly this, since it calls for engineering judgement in its execution and requires the orientation and direction of excitation to be stated and reported. Handled this way, you keep your certification clean and avoid a harsher horizontal test than is necessary.

Why mass changes the test

One detail catches suppliers out. The test profile depends on the mass of your equipment, because the frequency breakpoints that define the vibration spectrum shift with weight. A Category 1 item up to 500 kg is tested from 5 Hz to 150 Hz. As mass rises, that upper frequency drops, reaching 60 Hz for items over 1250 kg. The logic is sound, since heavier equipment responds at lower frequencies, but it means the test we run, the fixture we build, and even whether it is feasible on a given shaker, all depend on an accurate mass. Under-state it and the profile may be invalid. This is why mass is one of the first things we ask for.

What to send us for an accurate quotation

Most of the back and forth discussion for rail vibration quotations is down to a few missing details. To cost your test accurately first time, provide us with:

the mounting location on the vehicle (body, bogie or axle), or the EN 61373 category and class if your customer has already specified it
the total mass of the equipment, and its overall dimensions
The mass and footprint of the fixture.
A drawing or model showing the fixing points and the normal working orientation
whether functional monitoring is needed during the test, and a functional test procedure where possible.

With that information we can confirm the category, help design a suitable fixture, and give you a clear cost and lead time. If you are not sure about the category yourself, send what information you have and we will be happy to help.

Talk to Axis about your rail testing

Axis Test Laboratories is a UKAS-accredited environmental testing laboratory in Durham, carrying out shock and vibration testing to BS EN 61373 alongside the wider rail standards your customers ask for. Whether you have a fully specified test ready to book, or just a component and a question about which category applies, we are glad to help.

Get in touch to discuss your requirement or request a quote, or call the team on +44 (0)191 378 4653.

BS EN 61373 Category 1 vs Category 2: What Rail Suppliers Actually Need to Test