Here's what I've noticed on the east coast:

Ask politely and you'll be told "NO".
This when it's actually legal to be somewhere like
a platform or sidewalk.

Don't ask - and it's a 50/50 shot you'll be approached
- those that do, half are just curious and actually
doing thier job. The other half will hassle you to
no end - folks that didn't gradjitate high school but
have badges.

I'd be hard pressed to actually attempt to walk
(tresspass) into a yard, though in the past, I
was able to strole right into 30th Street Station
and take all the pictures I wanted.

The local tourist steam lines are the exception so
long as you do not behave like an iditot.

/Mitch