Robin I have just done a re-bogie of a 3 dome tank car by deleting the bogies in config and putting in thier place 3f6i bogies ( changed the kuid items) and it works! try downloading some 36in bogies and do a swap! That is what the Chineese and Russians do every day at the border, so it must work ! Good luck
I just can help you as you want 36in Oil tanker you can get it from the oil companies its depend on your requirement first you should have a license for that for the respective company from which you want oil then onwards you can proceed
