А почему есть третий рельс по середине?

This may have been a very local solution for the cabling.
You have to look at everything in a double context: the spirit of the times (the planning dates back to the 1970s) and the fact that the tracks were produced there. Money was not an issue at the time (another authority paid for it) and the line was of no avail: it went to places where one bus every half hour was and is more than enough.
The whole thing was considered an employment project and that third rail there in the middle must have been a good reason to give the metallurgists (and that was the main industry there then) some extra work.