`streets`' `rail=bool` value is unnessesarly included at high zoom levels

[CommanderStorm]

Currently, the rail property is incuded at zoom 5+. It is documented as

true for railways

THis does not make sense, as the first railways=* feature (rails with service) will show up at 8+.

Assuming that kind=motorways can not be a rail.

I would like to propose to include this property only at 8+

[pnorman]

I'd rather drop this field completely. We should either have users

• use kind to determine if something is a railway or
• drop kind and have highway and railway fields

also related #44