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Driving a DMU

Preparation and Basic Technique

(Compiled by Stan Acaster (Dean Forest DMU Group))

Keys Required

Brake Handle, Reverser, Control Isolating Switch Key, Carriage Key, Corridor Door Key.

Switches to be Closed before Engines can be Started Locally

Main Battery Switch, Control Isolating Switch, Engine Isolating Switch. If the latter is switched off find reason.

Control Isolating Switch, when Main Battery Switches are closed, gives through control current throughout train.

Reverser makes desk alive and selects direction, enabling current through C.I.S. to be used to operate Controls from Cab.

Deadman's (throttle control) must be engaged, as this in turn energises the Gear Selector.

Preparing Car

These preparation instructions are written for a 1 power car and 1 trailer car combination. The preparation is written assuming that the Unit is to be driven from the cab of the power car. Curent practice on the Dean Forest Railway is to leave the DMU with the final drives isolated. Before any attempt is made to start the engines the driver should satisfy himself that the drives are isolated by checking the position of the isolation plungers and that the cardan shafts to the final drives are free to rotate.

Proceed to cab which is to be driven from, see hand brake is on, flags and detonators, hand lamp with a red shade and trimmed, fire extinguisher is intact, deadman's is not isolated, no keys left in desk, A.W.S. flag in use.

Place tools on desk, insert and turn C.I.S., place reverser in pocket, proceed down No. 1 side, check that speedometer drive cable is retained with a strap or chain, continue along the side and turn on battery switch. On BUT/Leyland engines, examine sump first.

Depress fire bell test button, observe that red solebar lamp illuminates and fire bell rings in Cab. Then pull out throttle control and press start button, five seconds after engine fires, release start button, and gradually reduce throttle till engine ticks over. Examine belts, fuel, fire bottle connections, springs, brakes, jumpers, vacuum hoses and air pipes.

Check that corridor connections are secure and correctly engaged. Walk round front to engine on other side. On walking back to the front of the car "crack" all drain cocks on air receivers and drain any condensation that may have accumulated. Ensure all drain cocks are closed before departing. Start engine, then walk down cars examining as before. Walk round rear cab examine check speedometer cable has a retaining chain/strap, then enter rear cab, release hand brake, carry out cab drill as before, close windows, lock doors. Walk down the other side examining, glance in cab and see air pressure is 75 p.s.i., proceed to first engine, examine for leaks, then, if in shed, stop by stop button, if outside, stop by testing the fire alarm with test switch. On A.E.C. engines examine sump, stop other engines the same, and when passing guard's van pick up the isolating fork proceed to power bogies and engage final drives. Return to the Driving cab and operate the reverser two or three times, ensure that the air axle ligh extinguishes and re-illuminates, listen for final drive dog engaging. Remove CIS and reversing lever in that order. Inspect cardan shafts to both final drives and check that the drives have engaged. It should not be possible to rotate the cardan shafts by hand. Return to cab, insert reverser and put into the last selected position. Insert CIS and turn on.

Press start buttons individually and start engines. If necessary, engage Deadman's and open to throttle one to assist.

Create vacuum and see if it stands at 20"-21" with valve in lap, destroy 5" to test I.R. glands, then release Deadman's and see if it operates. Check all braking systems, deadman's braking system and deadman's override system. Give Guard vacuum test.

To Move Off

Apply vacuum brake, release handbrake, create 15" of vacuum, select No. 1 gear, then immediately release brake and when car moves open throttle.

When taking to cars that have stood for a time, or brakes have been adjusted or blocks changed which entails operating cylinder release cords, the train pipe vacuum must be created for 1.5 minutes to make sure that a vacuum has been created in the chamber (top) side of brake cylinders. There is no reading for chamber side except on single-car units.

Changing Gear and Gear Speeds

No. 1: 1-15 m.p.h.
No. 2: 15-27 m.p.h.
No. 3: 27-41 m.p.h.
No. 4: 41-65 m.p.h.

When maximum 1,800 revs. are reached, close throttle, let revs. die to 1,200 r.p.m., select gear, let revs. die to 600 r.p.m., open on throttle 2. When engine has built up to road speed, i.e., tachometer finger comes to a stand, open throttle wide.

When about to coast, shut throttle, let revs. die down to below 1,400 and select No. 4 gear. Coasting must not be done in neutral, neutral must be engaged just before coming to a stand. When it is required to change down through heavy gradient, when revs. fall back to 1,400, shut throttle, and immediately select the lower gear. When revs. reach 800, open on throttle 2 and possibly leave it there with maximum engine revs. and maximum speed for that gear and economy in fuel. It is a good practice to let revs. fall to 1,100 r.p.m. and pick up at 600 r.p.m. when changing from 1 to 2.


When in an intermediate gear, allow revs. to fall to idling before selecting gear 4.

Prep and Basic Technique ->
General Info
Do's Dont's and Remembers
What do I do if...?
Fault Finding
Jumpers Disconnected

Driving a mechanical transmission DMU meant changing gears, those that had hydraulic transmission didn't (unless they were coupled to a mechanical DMU).

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