What are bankers? Why are bankers used?
The above pics are showing some Wag5/7 locos working as a Bankers on Ghat section.These pics are only for a example of Bankers.
A banker is a locomotive that assists in hauling a train up a steep gradient. A banker is attached to the rear of the train and pushes the train from the rear while the normal locomotive of the train pulls it as usual from the front.
Bankers...
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are used for two reasons. One is that, of course, the leading loco may need assistance on a steep gradient. However, a more important reason to have a banker at the rear when ascending a grade is to protect the train from a possibility of coupling failure and consequent parting which would cause a portion of the train to hurtle backwards because of the gradient (guard's brakes being generally inadequate for such a situation). When descending a grade, bankers may be attached at the front to provide extra brake power (or sometimes just to allow the locos to be returned to their shed after having banked trains up the grade earlier, without taking up a separate slot on the timetable).
On an incline, when the train is being pulled up, the couplers come under a lot of strain. Normally, on level track, the couplers only have to sustain the forces corresponding to the static and rolling friction of the wagons or coaches. But when being pulled up, a component of the wagons' or coaches' weight also forms a part of the load on the couplers (the proportion of the weight arising from the sine of the angle of the gradient). Hence, there is a much higher probability of coupler failure when going up an incline. Finally, the additional locomotives help contribute extra brake power for the rake on the slope.
Often two, three, or even more banking engines may be provided on particularly steep grades and for heavy freight loads. It is common to see 3 rear bankers for passenger trains with 21+ coaches. On the Igatpuri-Kasara section even descending trains get two or three front bankers. It is common to see the Kushinagar Exp. get two WCG-2 bankers and a WCAM-3 up to Kasara.
Other trains on the same section often get three WCG-2 locos banking in front of a WCAM-3 when descending. Sometimes, however, bankers are attached to trains simply because there are available locos that need to be returned to one shed or the other, and using them as bankers is a way to move them rather than sending them light and reducing track utilization. The working timetable for a division specifies the local rules in effect for how many and what kinds of locos to use as bankers for different kinds of trains and loads.
The safety requirements for train operation set forth by the Commissioner of Rail Safety forbid operating passenger trains on steep gradients without bankers. Goods trains are sometimes operated on such sections without bankers if loads are light. EMUs are sometimes moved between Pune and Mumbai for maintenance and no bankers are used in such cases on the ghat sections as they are not carrying passengers.
The specific rules for what inclines necessitate bankers may vary from one zonal railway to another. In addition, bankers must be used for gentler inclines if there are special circumstances such as operation without brake vans.
The limits on the tensile force the screw coupler can handle necessitate the use of bankers for most Mail or Express trains these days even on fairly gentle gradients of 1 in 60 or so, since the rakes have been getting longer (and therefore heavier) in recent years. Hence the Nagpur-Itarsi ghat section requires bankers for all passenger trains with 18 or more coaches. Many trains with 17 coaches are run through on the ghat section for fear of overstressing the couplers if a stop is made and the train has to start on the incline. With CBC couplers, the allowable tensile loads are far higher. Goods trains with CBC couplers often don't need bankers on slight to moderate inclines for train parting reasons, but may require bankers to assist the leading loco.
Often, brake vans are removed from the rake before a banker is attached at the rear, because the common 4-wheeled brake vans are light and do not share the same mass/inertia characteristics of the freight wagons, causing them to be jolted around excessively and very often jump the rails due to the buffing action between the wagons and the banker locos. A newer, long 8-wheeled brake van has recently been developed which may avoid this problem, at the cost of making the rake longer.
In addition to the use of bankers, ghat sections often have special rules of operation. Mandatory brake halts are provided for steeper grades so that a brake power check can be done before the train proceeds on to the grade. Stopping at the top of a grade before descending also ensures the train is under control before proceeding. In steam days it was often common, for the steeper grades, to inspect all the brake cylinders of the rake at the mandatory brake halt, with defective ones being replaced immediately. There are also timed signals provided in some places; the train must stop at the signal for a specified time before it goes off and the points switch away from the catch siding, ensuring that only trains able to come to a halt there can proceed. 'Auto Emergency Brakes' are provided for locos intended for use on several ghat sections. These apply the brakes automatically if the speed exceeds a certain threshold.