Also, if you do come to Doncaster one of the local bus companies has a Routemaster(the iconic London bus). It gets used a lot on weddings but if you fancy hiring it for a transfer from the station to your hotel or just for a general ride around I'm sure they'll be happy to oblige!
http://www.wilfreda.co.uk/coach-hire/routemaster-london-bus
You needn't go to Doncaster for a Routemaster - Why not travel on one it its natural environment?
If you are in London i would suggest route 15 by bus ( Trafalgar Square to the Tower of London) Much as i hate buses the Routemaster that runs on that route is a fantastic historic British design.
The routemasters (
http://www.flickr.com/photos/darlo2009/sets/72157632327123219/ - in case you want to see what one looks like) run on route 15 (and a few outhers in London) every 15 minutes between 10 - 5 every day. That route 15 is fantastic and shows off some historic buildings.
You can also visit the London Transport Museum ( at Covent Garden on that bus route) which has a great deal of railway related items, although mainly to do with the underground. this did, of course, start under steam power 151 years ago today

I guess you will want a run on the underground to
The oldest part is the Farringdon to Paddington section of the Metropolotan line. You might want to compare that with the newest which will be the jubilee line between Westminster and Stratford. With a bit of planning you could get in the bus, tube, cable car, DLR and River in one day on an Oyster card.
Some pictures here:
http://www.flickr.com/photos/darlo2009/sets/72157632245374509/ and here:
http://www.flickr.com/photos/darlo2009/sets/72157629311023464/
I am sure some more ideas will pop into my head!
If you are in London i would suggest route 15 by bus ( Trafalgar Square to the Tower of London) Much as i hate buses the Routemaster that runs on that route is a fantastic historic British design.
The routemasters (
http://www.flickr.com/photos/darlo2009/sets/72157632327123219/ - in case you want to see what one looks like) run on route 15 (and a few outhers in London) every 15 minutes between 10 - 5 every day. That route 15 is fantastic and shows off some historic buildings.
You can also visit the London Transport Museum ( at Covent Garden on that bus route) which has a great deal of railway related items, although mainly to do with the underground. this did, of course, start under steam power 151 years ago today

I guess you will want a run on the underground to
The oldest part is the Farringdon to Paddington section of the Metropolitan line. You might want to compare that with the newest which will be the jubilee line between Westminster and Stratford.
With a bit of planning you could get in the bus, tube, cable car, DLR and River in one day on an Oyster card
Some pictures here:
http://www.flickr.com/photos/darlo2009/sets/72157632245374509/ and here:
http://www.flickr.com/photos/darlo2009/sets/72157629311023464/
I am sure some more ideas will pop into my head!