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When stock is moved around the network for refurbishment etc, it always seems to be accompanied by "barrier vehicles" which are usually knackered old carriages coupled between the loco and the stock to be moved although I have seen oil tankers used for this also in at least one photo in a recent Rail. 

What are barrier vehicles for?

I had assumed that they were to allow incompatable couplings to be used (ie as an "adaptor") but now I think more that can't be a full explanation as for example a class 57 pulling refurbed mark IIs or IIIs also often has these barrier vehicles despite having, I assume, compatible couplings.

Explanations greatfuilly received. Thanks

'Barrier vehicle', although you'll see it used in nearly every rail mag or on most enthusiast sites, is the wrong term as far as I know.  What you normally see are a mixture of translator vehicles (to deal with conflicting coupling issues, eg you can't easily couple a 66 to an Electrostar), or 'brake force runners', which are used when the dead in train EMU/DMU or other rolling stock being moved has its own brakes out of use, or uncontrollable.  The idea is that the braking force of the loco and the various 'extra coaches' is adequate for the overall train's mass.

A perfect example of this recently was the move of LU 'S stock' (you've possibly seen the photos?) where the train consisted of top and tail Class 20s, a few ballast wagons, tanks and an 8 car underground train. You can imagine that an ancient Class 20 and a brand new underground train don't exactly have compatible control systems.

A 'barrier vehicle' was originally an extra truck in a goods train, used to maintain a safety gap between the loco or brake van and a dangerous cargo such as explosives.

Paul

thanks for the explanation.  I'd forgotten about the brake-force issue.

I think the photo I saw was of brand-new Underground stock on the national network.  I guess tanks (presumably full of water) and ballast wagons were there to give extra weight to the train.   

When the 20s and 25 s first came out they had such poor braking especialy on the unfitted frieghts around at the time that they quickly decided to couple the 20s in pairs usually nose to nose and hastily contructed some brake tenders on the underframes of old coaching stock to be used with the 25s and other early diesels. Basically they were a large lump of steel on old running gear. I used to see them coupled to the cross London freights at Acton Yard.

Now of course you've got the whole issue of the vast number of incompatiable couplings in use on the current railway.

Useless fact: Did you know the USA with it's mulitude of private railway companies standardised the buckeye coupling and airbrakes from around 1910?

Only partly true I'm afraid - certainly loco-hauled stock in the States is fitted with buckeyes and air-brakes as standard, but then loco-hauled stock in the UK is all compatible as well through the use of drop-head buckeyes and screw couplings.

There's an eclectic variety of diesel and electric MU stock here as well though, owned and operated by a wide range of state and country railroad administrations, and all with the same coupling/control gear incompatibility issues as in the UK!