I was on a train recently and I tried to plug my macbook plug into the socket. Nope. The depth of the plug was too great. As it happened I had a 4 way adapter in my backpack so I was able to get power anyway!
Update! Thanks to Phil's suggestion I checked my cable drawer and found the other cable :-) I popped it into my laptop bag and found myself on a GWR train again yesterday. No need for a 4 way adapter this time (which I didn't have anyway - I only carry that when I'm running a CyberDojo).