Nick,
I think that because the series was specifically about transport, the efficiency for steam powered engines was quoted to be so low was because the engine was exhausting the steam to atmosphere. With a condensor however, the efficiency will improve.
A figure of 38% for a steam train seems a bit high, as that, as you say, begins to enter the realms of the power stations, and re-heaters. With all the gizmos in place, in theory I believe in htis modern age up to 90% can be achieved.
Anyway,
In trying to locate the programmes transcript, would you believe that Robbie Coltrane played rugby union for Scotland’s schoolboys !
SS