Q. What's the ‘tractive effort’ of a locomotive?
The ‘tractive effort’ is a measure of how large a load the loco can pull and set in motion from a standstill — the maximum force it can exert at the drawbar or coupling. While the raw horsepower rating of the loco is important, it is not the whole story. The loco's weight also comes into play, as a heavier loco can pull a larger load without its wheels slipping. Once the wheels begin slipping, the force that can be exerted by the loco drops dramatically. (Slipping occurs more with the front wheels because the front of the loco tends to lift slightly due to the reaction torque exerted by the rails on the loco.) Modern locos tend to have electronic slip control to control the power applied to each axle separately to minimize slip and maximize the tractive effort under different conditions.
Q. What's the method that IR uses to couple together multiple locos? or, What's ‘Locotrol’?
Two to four or so locos can usually be coupled together to operate automatically, without any special provisions, with the crew manning only one of them. (This mode of operation is known as ‘Multiple Unit’ operation, or ‘MU’.)
Using more than about 4 or 5 locos together without some form of automatic control for them is problematic because couplings come under excessive strain and break. A system known as ‘Locotrol’ is used on IR, which couples 3 to 5 locos at the head of a train, and one or two somewhere in the middle of the train, and possibly another two or three at the rear. The locos at the middle and rear of the train are radio-controlled by the crew at the head of the train. This system is used in some places for heavy freight sections. It was introduced with WDM-2's on the Kirandul-Kottavalasa line (near Vishakapatnam) in 1988 by SER. That line is now electrified and multiple WAM or WAG formations are used instead. Locotrol is (or was) also used on the Kulem-Londa section.
With multiple locos lashed up for MU operation, there are MU cables that pass between successive locos, which carry the control signals between the manned loco and the others. WDM-2 locos have a single MU cable, whereas the older WAG and WAM series use three MU cables, which also carry two connections for the tap-changers in the locos. The newer WAP-5 and WAG-9 locos use a single MU cable. The absence of this MU cable indicates manual coupling of locos -- in this case, each loco is manned by crew, and they use horn or hand signals to communicate across the locos. Such non-automatic coupling can also be used for incompatible classes of locos, e.g. to couple a WDM-2 with a WAG loco in rare instances where the required electric loco wasn't available. Even with some of the newer locos, coupling may not be possible for different classes of locos: e.g., a WAG-5 and a WAG-7 cannot be MU'd together and require separate crews for operation.
In some cases the local loco sheds or workshops have carried out experimental modifications to locos to allow multiple unit operation even in cases where the locos were not designed for MU'ing originally. The Moula Ali shed of SCR, for instance, carried out an experiment in the 1990s to MU WDS-4 shunters to allow MU'd pairs of these to shunt long (24-coach) passenger rakes (SCR had an especially high number of 24-coach train services and not enough shunting power to match). Normally a single WDS-4 can only shunt up to about an 18-coach passenger rake. The experiment with MU'd WDS-4 did not work despite a lot of improvization and experimentation by Moula Ali and eventually this was dropped, as old WDM-2's, and even WAM-4 or WAG-5 locos started being retired from mainline operations and becoming available for tasks like shunting.
IR never adopted the practice of designating locos as ‘A’ or ‘B’ units for coupling. All locos could be used as either a master or slave loco in coupled configurations. A few pairs of locos were sometimes kept in fixed-formation (mostly WDM-2 pairs); in such cases, both locos in the pair are usually oriented with their short hoods facing outwards (i.e., so that the short hood leads no matter which direction the locos travel in). (This is not universal practice, however, as lash-ups the other way around have been seen.)
Two to four or so locos can usually be coupled together to operate automatically, without any special provisions, with the crew manning only one of them. (This mode of operation is known as ‘Multiple Unit’ operation, or ‘MU’.)
Using more than about 4 or 5 locos together without some form of automatic control for them is problematic because couplings come under excessive strain and break. A system known as ‘Locotrol’ is used on IR, which couples 3 to 5 locos at the head of a train, and one or two somewhere in the middle of the train, and possibly another two or three at the rear. The locos at the middle and rear of the train are radio-controlled by the crew at the head of the train. This system is used in some places for heavy freight sections. It was introduced with WDM-2's on the Kirandul-Kottavalasa line (near Vishakapatnam) in 1988 by SER. That line is now electrified and multiple WAM or WAG formations are used instead. Locotrol is (or was) also used on the Kulem-Londa section.
With multiple locos lashed up for MU operation, there are MU cables that pass between successive locos, which carry the control signals between the manned loco and the others. WDM-2 locos have a single MU cable, whereas the older WAG and WAM series use three MU cables, which also carry two connections for the tap-changers in the locos. The newer WAP-5 and WAG-9 locos use a single MU cable. The absence of this MU cable indicates manual coupling of locos -- in this case, each loco is manned by crew, and they use horn or hand signals to communicate across the locos. Such non-automatic coupling can also be used for incompatible classes of locos, e.g. to couple a WDM-2 with a WAG loco in rare instances where the required electric loco wasn't available. Even with some of the newer locos, coupling may not be possible for different classes of locos: e.g., a WAG-5 and a WAG-7 cannot be MU'd together and require separate crews for operation.
In some cases the local loco sheds or workshops have carried out experimental modifications to locos to allow multiple unit operation even in cases where the locos were not designed for MU'ing originally. The Moula Ali shed of SCR, for instance, carried out an experiment in the 1990s to MU WDS-4 shunters to allow MU'd pairs of these to shunt long (24-coach) passenger rakes (SCR had an especially high number of 24-coach train services and not enough shunting power to match). Normally a single WDS-4 can only shunt up to about an 18-coach passenger rake. The experiment with MU'd WDS-4 did not work despite a lot of improvization and experimentation by Moula Ali and eventually this was dropped, as old WDM-2's, and even WAM-4 or WAG-5 locos started being retired from mainline operations and becoming available for tasks like shunting.
IR never adopted the practice of designating locos as ‘A’ or ‘B’ units for coupling. All locos could be used as either a master or slave loco in coupled configurations. A few pairs of locos were sometimes kept in fixed-formation (mostly WDM-2 pairs); in such cases, both locos in the pair are usually oriented with their short hoods facing outwards (i.e., so that the short hood leads no matter which direction the locos travel in). (This is not universal practice, however, as lash-ups the other way around have been seen.)
Q. What's the cause of the characteristic jerk or momentary loss of power felt when a WDM–2 accelerates?
The WDM-2 loco, like most diesel-electric locos, has several configurations of its traction motors that are used as the loco accelerates from rest (Series Parallel - Series Parallel Shunt - Parallel - Parallel Shunt, and weak field configurations). There are three important transitions: At 30.8-39km/h 2S-3P Full Field to 2S-3P Weak Field, at 48-55km/h 2S-3P Weak Field to 6P Full Field, and at 88-90km/h 6P Full Field to Weak Field. For the transition at 39km/h (it can actually happen anywhere between 30km/h and 45km/h), the generator's fuel supply is cut and it is momentarily switched off to avoid sparking and strain on the switchgear. This momentary loss of power can usually be felt throughout the entire train, and is very characteristic of this loco. It is much stronger than the jerks or blips in the acceleration felt at the other transitions of the motors. (This is also seen in other similar Alco models; railfans have reported the same distinct jerk in Alco diesels used in Greece, for instance.) The WDP-2 loco has one transition at 55-62km/h although it is not quite as pronounced.
