Deprecate `rail-symbol`

[jamiemlaw]

It might have slipped through the cracks, but rail-symbol should probably be treated as a brand logo and therefore be marked as deprecated.

Rail Symbol 2 - Design Manual

Copyright
Rail Symbol 2 is a registered trademark of the Department for Transport.

[jguddas]

We could also make the name point to the train-track icon but I feel like that's quite a jump.

[jamiemlaw]

We could also make the name point to the train-track icon but I feel like that's quite a jump.

Yeah, that's a little bit of a stretch.

[ericfennis]

We should handle this as a brand icon and deprecate it