Booster Transformers

the boosters in AC traction are balancing transformers between line and earth to even out the sulpply.

I am going to have to dig out some old books here but what you are talking about is a' negative booster',from memory this was a rotating machine similar to an mg set,a shunt wound motor fed from the dc feeder coupled to a series wound generator designed to boost the negative ie running rails to 0v with respect to earth.board of trade specified (shoot me down in flames please) a maximum of 6v +ve or-ve between rails and earth.This of course applied also to tube lines and hence the fourth rail.

I can see that my OP was muddled and now that three knowledgeable members have answered it seems that 'booster' can mean at least three things: a booster transformer used on AC overhead systems to reduce the above ground voltage of the running rails; boosters being the machines used in DC locomotives to get them past gaps in the traction current rails; and negative boosters (what my OP had been struggling, in its confused way, to find out about) which are motor generators boosting the negative path of DC rail return systems to reduce unwelcome side effects, such as electrolysis of pipework. Perhaps negative boosters, or their modern solid state equivalents, are still at work all around the Southern Region and in modern tramways.

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My understanding is that in the days of LT trams electrolysis was quite an issue for the GPO whose cables at the time were lead sheathed. Piles of lead deposits were often found in the bottom of footway boxes and roadway manholes as a result of stray return currents. I believe that even today it remains standard to fit sacrificial anodes where stray currents from various sources could still be an issue.
On the underground until the 1980s there was no single earthing strategy and it was a big issue when I was installing cameras on stations at the time as the difference in potential between earths could be as much as 630 volts and this often manifested itself as interference to pictures and their quality. In those days the cameras were powered from the 'domestic' 240v mains supply which had a different earth to surrounding metalwork which could be at a different earth reference altogether depending upon what it was bonded to. Signal and traction supplies are generated earth free and tied to earth locally as a reference in each area, the areas being electrically isolated from one another as completely separate zones. In signalling in particular it is very important to ensure that each 'controlled' area is electrically separate from adjacent areas to maintain the integrity of the safety signalling circuitry.

Just to add to your list of boosters, signalling does use booster transformers in some locations where the length of cabling in a circuit would result in a significant drop to the nominal 100v ac local signalling supply. The booster transformers are auto transformers mounted at trackside locations on some lines such as the Piccadilly and District where signal selection circuitry can run back and forth for miles. While this is not the same type of 'booster' that you were asking about it is worth noting that like many things one word at LUL can have multiple meanings. Indeed there are also such things as track circuit 'boosters' which detect rainfall or other water level and automatically boost the capacity in a track circuit to ensure it continues to operate under a change to altered electrical conditions such as were used on the East London line.

As a postscript, here is a link to a site that gives more detail about boosters in general.
www.most.gov.mm/techuni/media/EP_03061_7.pdf

Yes, it takes a bit of persistence. I had to refresh about five times before it downloaded fully, using Chrome too.. It is worth it, though. If you have no luck at all, PM me and I'll e-mail it to you.