Best museum in London for trains for 5 year old?

[Terramirabilis]

We'll be in London for two days later this year from overseas and have a train-mad soon to be 5 yo DS. We can make it to only one museum/location for trains for him during our stay.

Anyone know which is likely to suit him best? Possibilities I'm weighing are Kewbridge Steam Museum, the London Transport Museum and the Science Museum. But perhaps there are others I don't know about.

We will also be using the tube so he will get a bit of a fix from that.

I've lived in London in the past so no worries about getting around, navigating buses etc. I just haven't spent much time in train museums previously!

Bump.

I'd definitely say the London Transport Museum of those, but do have a look at the Postal Museum and Mail Rail as well - you get a very cool ride on a tiny underground train.

Thanks!