That last bit is the problem. You can not simplify the check for every loco on every tick, in which network it currently resides. Just because most people just build one large network doesn't mean the engine can skip the check. (Also having power run through the rails would be wildly unrealistic and dangerous :p)
@Bob:
Accumulator cars too? Hm. I'd have thought that a train with accumulators (built-in or seperate carriage) that only recharges when stopped at a station would have at most the same overhead as fuel inserters. Do you remember the exact counter arguments for that (Except coding effort :p)?