Push Pull Control Systems

Agree with this I too have learnt a lot but one small thing I do want to know and it is not directly related to control systems. Why did the GWR and later the WR use the light engine headlamp code on their auto trains. The only exception to this I have found is the Exeter -Dulverton service where a ll the pictures ive seen its the normal Class B headlamp.

 

This came up on the GWR E list very recently:
https://groups.yahoo.com/neo/groups/gwr-elist/conversations/messages/65540

 

On the "Swanage Railway General Discussion" thread, there was talk of the possibility of an SR-type M7-hauled air-operated push-pull set for use on that railway. That discussion broadened into wider issues of push-pull operation, so perhaps should be linked with the earlier comments on this thread.

The GWR seems to have been alone in sticking with a purely mechanical push-pull system, which it deployed widely and successfully. In the Pre-Grouping era, mechanical push-pull systems were also employed by the GCR, LNWR, LSWR and SECR - and possibly others. But these systems were all found wanting and were replaced in later years by compressed-air and vacuum based systems. Yet the GWR system has survived into the preservation era, unlike the others.

Vacuum controlled push-pull gear was fitted as standard to all of the BR 2MT 2-6-2Ts. I assume this was based on the LMS system, which was fitted to many of the Ivatt 2-6-2Ts as well as earlier engines of Midland/LNW/L&Y origin. The RCTS account of the BR 84XXX states that the push-pull gear "operated through auxiliary regulator valves fitted in each steam pipe from the superheater header to the cylinders. These were single seated poppet valves and were operable from a compartment at the opposite end of the train via supplementary train pipes and controls".

Presumably, this "auxiliary regulator valve" was opened and closed by the presence or absence of vacuum in the additional vacuum pipe between loco and driving trailer? And presumably the main regulator has to be left open to allow remote control via this auxiliary valve?

Loco Nos 84020-29 were initially allocated for use by the SR in Kent, where of course they had the wrong type of equipment for the SR air-operated push-pull sets! The Bluebell Rly is converting a 2MT 2-6-0 to a replica 2MT 2-6-2T, which I believe is to be 84030. A point of detail for the builders is what to do about the alien push-pull gear carried by this class - do they fit dummy gear to match the appearance of the original engines?

 

I posted this in the Swanage thread:
In 'Locomotives of the Isle of Wight Railways', by Messrs. Cooper, Faulkner, Maycock and Silsbury, there is a schematic of the pull-push equipment on both loco and driving carriage. The main reservoir supplied air to a back-pressure reservoir which connected to one side of the piston in the regulator cylinder, tending to keep it closed. a higher pressure on the other side of the piston would open it. There were 3 air lines and one electric connection (exclusive of the brake pipe) - Regulator control; Main supply, Back-pressure supply. The principal is probably straightforward, the devil is in the details of all the doo-hickery that regulates pressures, &c.

The survival of the GWR system may be due to the relative lack of value in the components. The SR air pumps may have formed a pool of cheap spares for the Isle of Wight, and I would be surprised if either vacuum or air systems didn't have some copper piping in there.
Pat

 

I think apart from luck of the draw, its easy to see why there are still GWR Auto Trains in service and not any of the other Big 4

 

The utter simplicity of it as well must be a factor. None of this faffing around with air pumps, reservoirs, pipework, valves, etc. Just a few bits of metal bolted to the bottom of a coach and a flexible coupling to the loco. Very simple, very agricultural, but effective (for a couple of coaches anyway!).

 

My only steam ride in 1969 was Buckfastleigh to Staverton Bridge and that was a four coach push-pull operation - with a pannier tank (6412) sandwiched in the middle. Didnt make for good photography but interesting to look back on now.


Peter

 

I see this is quite an old thread. Noting the earlier comments about the French 141TCs, Devernay's "La Locomotive Actuelle" has a short chapter about this titled "Réversibilité", first two pages attached.

 

Because it's easier to put the lamp on the middle bottom bracket than the smokebox. This was quite common for WR branch working. It may have had something to do with the height of the fireman!

 

There's a bit more to it than that but, like most GWR designs, it worked so why change. However it was also common practice to leave the rod disconnected and the fireman operated the regulator. It wasn't totally bomb proof.

 

If you've ever tried putting a lamp on the top lamp bracket on the bunker of a GW, you'll know.