Originally Posted by
chinook9
I don't believe it is necessary to take the train which is outside the terminal.
Last fall we flew in from DEN to ORD on AA connecting to BA out of the international terminal. There is a bus that is operated specifically to take passengers between these terminals (3 and 5 I believe).
This bus may be operated only for AA/BA and IB connections but I am not sure. There were very few signs for it. The bus was on the tarmac, inside security, the whole time. It seemed like a real informal operation but it was functional.
Even if that's the case, it will be of no use to someone like OP who arrives on an international flight in T5 and needs to connect to a domestic flight in T2. After clearing immigration and customs, you are dumped into the public area of T5, outside security.
My understanding is that shuttle is/was operated by BA for passengers arriving on American in T3 and connecting to the LHR flights departing out of T5. As I live in Chicago, I've never taken it so I'm not sure if it still runs... but you can be sure it is only possible to use when doing a domestic to international connection and not the other way around.