On Thu, 8 May 2008 10:03:59 -0700 (PDT), 1506 wrote:

Moreover , it seems possible that having bought a drink at Marylebone
one could travel to Amersham whilst enjoying a drink on a Chiltern
train. If one bought a drink at Baker Street one would not be allowed
to consume it whilst travelling, over the very same metals, on a
Metropolitan line train.

Presumably if one were waiting on the platforms at Harrow-on-the-Hill
station for (e.g.) an Uxbridge service, and one wanted to take a sip
from one's hip flask, one would have to wait for a Chiltern train to
arrive, hop on, take a swig, and hop off again.