First let me say I have not found an easy fix. The axle is very complicated and I used a wheel puller and a gear puller, not tools in the average persons kit.
I started woth some bevel gears I found at Aliexpress.
(http://www.largescalecentral.com/FileSharing/user_174/PeterTs%20Folder/Railtruck/20210122_121014_bevels.jpg)
The gear had to be lightly sanded and the width reduced. Then I drilled it out 3mm and filed the holet to an oblong shape as both axle and driveshaft have flats and are 3.2mm diameter.
(http://www.largescalecentral.com/FileSharing/user_174/PeterTs%20Folder/Railtruck/20210122_175451_cutting.jpg)
The gear puller took the gear off the driveshaft:
(http://www.largescalecentral.com/FileSharing/user_174/PeterTs%20Folder/Railtruck/20210123_151304_gear-pull.jpg)
The wheel puller was used to get the wheel off. NOTE that there is a tiny bolt in the end of the axle that must be removed, the hub taken off, and then the bolt must be replaced tightly into the axle as it takes all the strain of the wheel puller.
(http://www.largescalecentral.com/FileSharing/user_174/PeterTs%20Folder/Railtruck/20210123_152332_wheel-pull.jpg)
There's a full essay with lots more photos here:
http://www.largescalecentral.com/forums/topic/31279/bachmann-rail-truck-axle-fix-maybe (http://www.largescalecentral.com/forums/topic/31279/bachmann-rail-truck-axle-fix-maybe)