Train Handling
Train dynamics can be extreme and therefore an engineer must be familiar with train handling techniques so as to avoid train partings, derailments and not exceeding line speed.
Freight trains typically have different train forces from passenger trains. A typical freight train may have 500 tonnes of locomotive weight at the front. That may be followed by 1500m of wagons. The wagons may or may not be uniformly loaded and may brake differently.
Severe brake applications can combine with these factors to cause a train parting. Therefore good train handling practice for freight trains is usually to keep the consist (rail vehicles which make up a train) stretched. This is achieved by keeping the consist in power while a brake application is made and by bleeding the air off the locomotives brakes before they apply. It is not possible to do this with the use of dynamic brake, which presents its own train handling challenges.
When there are multiple locomotives, some may be set up to brake like wagons instead of locomotives, as too many locomotives on the front of the train (all with brakes bled off) would require too heavy an application from the rest.
On shorter passenger trains, this is even more noticeable, requiring the first application of the brake to be bled off on the locomotive, applying locomotive brakes with subsequent increases in application. The length and make-up of the slowing or stopping distance dictates just how much locomotive brake application should be allowed to apply.
The use of dynamic brake can result in a severe slack action, when engaged, run in is highly possible if brought in at an inappropriate time (regarding track geometry and train speed) and if disengaged at an inappropriate time can result in a run out. Both can potentially snap train drawgear or couplings.
Straightlining is a potential cause of derailment that train handling techniques must take into account in order to reduce the likelihood of occurrence. When a train rounds a curve basic physics dictates the trailing wagons in the consist will try to take the shortest route and the flange on some of the wheels within the consist could potentially fail to prevent this occurring with the resultant effect being a derailment.
Track geometry is also critical to train handling. It is desirable to have brakes releasing at the bottom of steep grades rather than applied. And at the top of a steep grade it is desirable to have a fully charged brake pipe.
Serial braking is where a train descends a grade on the air brake alone. The brake pipe application is gradually increased to slow down and if required (depending on the weight of the train and on the grade) stop the train so as to allow the locomotive compressors to recharge the brake pipe throughout the consist. In these cases it is permissible to use the locomotive brakes (which are independent of the train brake and charged through the main reservoir directly) to hold the train (In some cases the weight of the trailing consist will not be held on the locomotive brakes alone) slowing the rate of acceleration and giving more time to recharge the brake pipe to give a better application in the next subsequent train brake application. A runaway can occur if a brake application is required before the train pipe has recharged (as happened at Cima Hill in the United States).
A split reduction is where a train brake application is made and gradually increased as the train descends the grade. It is different to serial braking in that with Serial Braking the application is released, the brake pipe recharged then reapplied.
The dynamic brake when operable slows down the rate of acceleration and allows longer for a train brake pipe to be recharged before being required to be re applied. When a train descends a grade utilizing both the dynamic and air brakes the procedure is known as 'maintaining braking'.
In the case of severe grades (for example the Westmere Bank in New Zealand, which is a 1:33 grade with a 40 km/h (25 mph) speed limit) a trains allowable speed is lower for a train that doesn't have dynamic brake than for one that does.
In freight train marshalling yards the wagon brakes are sometimes bled off so they can be easily loose shunted. However when a shunt locomotive moves large numbers of wagons around with no brakes the locomotive must brake for the entire train. This can result in severe slack action and wheel slip. Damage to goods and rolling stock is possible. Also, with unbraked wagons there is potential for a runaway.
Read more about this topic: Railroad Engineer
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