Die Bahn and why not Powerline Communications?

I was thinking about internet access on trains, and the fact that it's still not available in the UK. In Germany, Die Bahn uses UMTS connectivity for internet access on its trains, and caches the contents of the T-Online portal at each station (in German; in English with Google). This has serious technological limits; why they don't use Powerline technology on the overhead pickups, I don't know - any takers?