Yes, there are some train companies which will accept tandems. We've taken ours on Southern and Southeastern officially, and also on London Underground with the tolerance of station staff at Kings Cross. We've also taken advantage of a very quiet station and a very quiet driver-only train to smuggle it on our local Chiltern Railways services, even though there isn't really room. But both our tandems have S&S couplings, so it's a matter of seconds or a couple of minutes (one takes slightly longer than the other) to turn a seven-foot monster into two normal-length bits if an on-train employee gets shirty. That's happened before now.