As well as wheels and axles (Alan Gibson brand recomended) they will need a chassis to complete.  4mm one's are available for £12 each although they are only a stop-gap to producing something more robust.  They work fine but in resin are a bit brittle so care is needed when threading wheels on to axles through the chassis etc, small 2mm spacing washers are a good idea too depending how much  lateral 'give' you want the wheelsets to have.  The chassis need a little filing before they fit well due to the MSLA printing process is all.
 
Tender completed by Andrew Postlethwaite