Question: I have 12v train 7740. If I update the wheels of the cars to a modern version, would that make a lot of difference in speed of the train?  Incan also run it with the sleeper car and the post car.

Edited August 2, 20231 yr by Good old Lego builder

You mean the steel axle wheels from the 9V/PF/RC era? yes, you could use those, alough you'll have to change the complete boogie design.

I use them on all my old 12 volt rolling stock, much smoother ride. 

Yes, that is what I have in mind. Thanks!

You could also keep the basic bogie design and get an even smoother rolling using the 3rd party (or homemade) roller bearing trucks. I think most of the folks selling them use the 1x2 technic brick with two holes, but at least some of them will happily use the one hole variant if you ask. The net result should be similar form factor to the 12v train wheels.