Top and tail

[the_viffer]

I've agreed to provide some motive power for a friend's garden railway at his "special" birthday party.

I'm planning to top and tail which is to say have a loco at either end of the train. The leading loco will provide the power but what do we do with the trailing loco? Can it stay in forward gear (I'd quite like not to be winding like crazy back and forth with the reversing handle)? Do I need to open the drain cocks on the trailing loco? Any other tips?

[waggy]

Don't leave the engine in any gear, must be in mid gear. If you leave in forward or reverse and drag the other way with the taps shut you will create a compressor, if the pressure can't get into the boiler where the safety valves can vent it, it may lead to compression lock up and cause who knows what damage. Leave the taps open, motion in mid gear and regulator shut.
Try it before you steam up, everything shut apart from regulator, forward gear, drag back and you will soon see the needle come off the stop!

As regards your gear when you drag it back, that's up to you!!

Waggy.

[chris vine]

Waggy is quite right but you will find that there is a lot of drag from a cold engine. Although the drain cocks are open there will be a lot of back pressure on the pistons if you are running at any speed. The holes through the drains are tiny compared to the main steam pipes!

Why not do the job properly and have both in steam with two drivers and whistle codes etc.....

Chris.

[the_viffer]

cvine said:

Why not do the job properly and have both in steam with two drivers and whistle codes etc.....

Chris.

We were going to do that but I had supposed that the tail of a top and tail was deadweight. I'll maybe select locos with pole reversers...

On an allied topic I'm proposing to run two of my electric locos double headed this year. I've sorted out the electronics for doing this. However I'm not sure which loco pulls away first and which slows first in such a situation. Realistically I'm not going to get the ramp rates identical. Can you help?

[waggy]

What does the viffer mean by "electric loco" ? Do they use coal from a power station?

In real life the two loco's would pull away together. They would be electrically coupled by jumper cables so would both take power together. If they were air or vacuum braked the lead loco would brake first and "gather" the train. This works because the driver destroys the brake pipe from the lead cab, the train pipe pressure changing from front to back.

In the case of an electro pnuematic brake the brake is applied to all vehicles at the same time as they all receive the braking command together.

HST's applied the brake from both power cars at the same time so the middle coaches were last to brake. They have air brakes controlled by the pressure in the brake pipe, an electrical command is sent to both power cars to apply the brake, hence breaking from either end.

Yawn, tired now!

Waggy.

[the_viffer]

Realistically you are not going to get both locos to pull and brake at exactly the same rate. One will be a tad slower than the other. This is especially true on steam locos when there are two drivers who simply can not consistently work the controls at the same time and rate.

Which one leads?

You've said the front loco brakes first. I imagine that the front loco pulls away first and takes some of the load on the draw bar before the train loco starts but confirmation or otherwise would be appreciated.

I'm thinking only of a small difference in time constants on the controllers but I bet one way round is better than the other and I'd rather be sure that I get the right way round.

[GeorgeRay]

In my time on the big ones I was told to put the engine in the direction it was going even with the regulator shut.
When using two locos top and tailing on a minature the one leading the train is the one to have control of the brake. I have done that with our portable track and its very easy, you just need to ensure that its the trailing loco that shuts off first.

[JohnP]

Going back to the double headed question, with two electric (or for that matter, any kind of loco) coupled together, they have to go at the same speed. What this means in practice is that the loco that would have been going faster takes more than its fair share of the load.
There is an exception to this, which is if the control system is sophisticated to the extent that if the loco speed is less than the demanded speed, the controller will keep on trying to get to that speed by passing more and more current. If this was coupled to a loco whose controller was trying to slow it down (rheostatic or regenerative braking), then the two locos would be fighting each other. Does anybody on the group know if that is the case?

JohnP

[waggy]

Which one leads?

In my experience of working full size electric loco's ( Class 76, 86, etc. ) in tandem they would both take power together unless the power contactors were a little sluggish one one of the loco's. I suppose the front loco would pull first but only by a fraction of a second.
The only way to make this happen is to fit some form of time delay in circuit which would delay the train loco taking power by whatever time you like.
Ideally you need to watch the shackle betwixt the loco's, tight and all is well, slack and the train loco is pushing the leader.

Waggy.

[Chris Kelland]

Hi,

I am interested in the prospect of the two electric locos running coupled, but what are they? Gauge, controllers etc? I run two 5 inch electrics permanantly coupled but with seperate controllers. The brakes are also controlled automatically with manual overide.

Regards,

Chris.

[the_viffer]

chriskelland said:

Hi,

I am interested in the prospect of the two electric locos running coupled, but what are they? Gauge, controllers etc? I run two 5 inch electrics permanantly coupled but with seperate controllers. The brakes are also controlled automatically with manual overide.

Regards,

Chris.

Chris a Class 13 which is effectively 2 08 0-6-0s permanentley coupled together and a pair of class 37s (ie Co-Co) one of which is very much more powerful than the other. They are in 5inch.

The controllers are 4QD. It is fairly easy to run some of these as master and slave so one handset runs two controllers. An advantage of this is that you can have a small control cable between the two locos. Braking is inductive. You can arrange it so that you can run double headed or replace the jumper cable with a handset for running single loco. I'm just at the stage of trying to set the time constants so that each loco has about a fair share of the load starting and stopping hence the thread.

If you want more details contact me on or off list.

[Tel]

waggy said:

Which one leads?


Waggy.

The Man!

[waggy]

John P is quite correct when he says both coupled loco's have to go at the same speed.
Regarding one loco in power and the other in dynamic braking mode (regen or rheostat ) this can't occur unless there is a major fault on one loco. If such a fault occured, I doubt you'd move never mind brake. In full size and ours, when in tandem both loco's control circuits are electrically connected by jumpers or auto coupler contact plates. I can't see anybody going to the trouble of trying to control two loco's seperately. The power or brake mode is selected by the driver, ( modern units have a system called brake blending, where the dynamic brake is called for automatically when the driver brakes the train, there is a balance between friction and dynamic brake set up ) you can't take power and brake at the same time. The exception to this is called the hill start brake, where the driver applies a partial brake and can take power at the same time so his train doesn't roll backwards on a rising gradient. This action usually controlled by a push button on the power handle, holding in gives a small brake application, the button and brake released when the unit moves off. The brake cylinder pressure typically around one bar, enough to hold the train but not to stop it moving when power applied.

An interesting subject, hope your not bored!

Waggy.