FAQ - Investigating Derailments

[LCDR]

DERAILMENT INVESTIGATION

The following is a guide to investigating and rectifying derailments on a model railway. I hope they will be of some use to somebody.

1  Faults with track

a     Rail joiners

Check that the rails are both snugly held by the rail joiner. The tops of the rails should both be the same level. Run a finger over the rail top surface. If you feel a lip or bump remake the connection again.

Check that both the rails are in line. Look along the track. There should not be any kink at the joint and the rails should be butting up to each other.

Do not use damaged or distorted rail joiners (AKA fishplates) a packet of replacement fishplates are an essential in the toolbox.

b.  Track level and twist

As far as practicable the all rails should both be at the same height, a small variation, especially through turnouts will encourage derailments. Look along the rails and check by eye that the rails look level and are parallel to each other. (N.B. turnout is the proper name for points)

Track which is not correctly pinned down on a level track bed may develop ‘twist’ which is when one rail rises on relation to the other over a short distance. It is a good idea to use a straight edge (steel rule) and a small spirit level to check.

It is permissible to have the outer rail on a curve to be slightly higher than the inner one but in OO scale this should never be more than 1mm. It is NOT permissible to have the inner rail of a curve higher than the outer one. The change in gradient track approaching any such increase on height should be very gentle. Recommended no steeper than 1mm in 200mm.

Beware of bowing through turnouts. The whole of the track through turnouts should be flat and level. A pin in the centre of the turnout unit will help to prevent it.

Do not pin the track down too hard so that it distorts. Pins should hold the track in place and keep it level but if driven in too hard may damage the sleepers and cause distortion.

c.   Turnouts not fitting up

The blades of the turnouts must always fit tightly against the rail. Even the smallest gap will cause a derailment. The most common problem with turnouts not fitting up is dirt, dust or grit becoming lodged between the point blade and the rail. Inspect them, and use a stiff bristle paintbrush to brush away and debris. A cheap artist’s paintbrush is ideal for this job.

Check that the operating mechanism is free to move when necessary, but that the spring that holds the point blade is sufficient to do the job and hold the blades tightly to the rail. Obtain and replace any weak or missing springs from the supplier. Damaged or defective turnouts need to be replaced. The point blades must never be higher than the rail.

d.  curves too sharp

Most OO models nowadays require a curve to be 438mm radius or greater. Only short 4 wheel locomotives and wagons are happy to pass round anything less. If using flexible track check and compare using a standard second radius curve that this radius is maintained or exceeded before pinning down.

e. transition to an incline, and curves in an incline

If your layout includes a raised section of track ensure that the incline to the upper level joins the lower level smoothly and gradually. Avoid having this transition on a curve if possible.

2. Faults with trains

a.          Wheels incorrectly spaced

‘Back to Back’ is the distance measured between the inner faces of wheels. It is recommended that this is 14.4mm +/- 0.05mm. Gauges are available from reputable model railway suppliers to check this but a Vernier gauge will do just as well. When using modern trains and track this dimension is critical because the flange depth will not prevent a derailment through turnouts.

b.           Mixed wheel standards

Avoid mixing unmodified trains and tracks of significantly different ages together. Wheel standards nowadays are considerably finer. Older Triang and Trix locomotives and rolling stock have deep flanges and require greater clearances through the crossing (frog) of turnouts. Conversely modern trains with finer flanges may derail on the wider gap present at the crossing nose of older turnouts.

The crossing nose sometimes called the ‘frog’; is the place on a set of turnouts where the wheels cross the rails of the alternative route.

By ‘older turnouts’ generally speaking track made before 1980 has much greater clearances to suit the coarser wheels of the time.

If you do wish to run older stock on modern track it is a good idea to replace the wheels with those of modern profile. Usually this is a direct replacement springing the old set out and springing the new one in. Very old vehicles may require some modification. Usually this entails inserting brass bearings into the axle-guards before fitting the new wheel-set.

Locomotives will require the attention of a skilled modeller or a professional. Replacement wheels can be fitted but this is precision work.

c.           Damaged or distorted wheels

The wheels should be round, and the flanges intact and not distorted. Burrs or blemishes will cause bumping and jumping that may lead to a derailment. Kit built wagons may sometimes have plastic wheels that are detached from a sprue. The burr left by the detached sprue may distort the flange. Carefully remove any extraneous material with a needle file.

Axles should always be at right angles to the wagon or the bogie frame. Wheels should always run true on the axle. If there is a noticeable wobble when the wheel is spun by the fingers the wheel set should be replaced.

The pin-point ends of the axle must sit squarely in the recess or bearing in the axle-guard and spin freely. It is a good idea to stand a vehicle on a plate glass sheet and check there is no perceptible rocking between the flanges. Any binding or rocking needs to be cured and if this cannot be done sadly the vehicle should be withdrawn.

d.          coupling compatibility

Mixed or damaged couplings can cause derailments especially when propelling (e.g. pushing with a locomotive at the rear). The horizontal bars of the coupling must be all the same height and must not ride up over the one on the next vehicle. The hooks must not tangle and should rest on the proper side. Bent couplings should be straightened or replaced. Where couplings are significantly different, consider changing them to a common type or introducing adaptor vehicles which present compatible couplings at opposite ends between the different types.

e.          Mixed weight vehicles

When propelling or hauling long trains around sharp curves or through sinuous point-work (for example entering sidings) any significant variation in the weight of individual vehicles may cause the lighter ones to come off the rails. The resistance of the vehicles is also a factor. If heavy vehicles are at the opposite end of the train to the locomotive, or the vehicles at the rear have tight wheels that are reluctant to turn, the force of haulage or propulsion will drag or push the lighter vehicles towards the centre of the curve and cause them to derail.

There are a number of ways this can be prevented.

• Try and keep all vehicles in a train to the same general weight in proportion to their length. Avoid lighter vehicles in the middle of trains.
• Avoid trains of excessive length, especially where the whole train exceeds the length of the curved section.
• Ensure all the vehicle’s wheels turn freely.
• Avoid sudden slowing and speeding up on curves.

3. Obstructions

Various obstructions to the passage of trains can cause derailments. Lineside structures such as stations, signals, tunnels etc. need to be clear of all the vehicles that may pass. This is especially important where long bogie coaches are used. These occupy much more width on curves than they do on straight track. The width available on curves needs to take into account ‘centre throw’ where the middle of the coach occupies extra space inside the curve, and ‘end throw’ where the ends of the coach sticks out at the outside.

It is not just fixed items that can cause problems. Other trains standing in sidings which are not sufficiently clear may be struck and cause a vehicle to derail. Also on parallel curves two trains passing each other may strike each other if the curves are not secured the proper distance apart. Always make sure you know that the train in the siding is well clear. A small mark at the clearance point may help to judge this if space is limited.

Operator or observer influence can also be a factor. A careless sleeve near the line may be all that is needed to cause a train to fall off. This is a common occurrence when attending to one train by hand whilst another train is still operating.

Then the occasional visit by the pet cat can result in a disaster.