Unless your two boys fit in the palm of your hand, this is the wrong place to post that question. Utilizing the drive train and battery from a fisher price power wheels might be something to consider. Other than that try google.
Do a google search for the phrase “riding toy train”. There are a couple hundred possible entries. Perego (the maker of ride on/in cars) makes a unit that might fit your needs. This is just a guess as you did not give their ages.
I did build a wooden “ride on loco” for my son when he was younger -he is now 10 and 165cm tall… The plans came from “Hobbies”. The only source of motors and controllers that could do the job is here. There must be a US equivalent(?) http://www.technobotsonline.com/ regards ralph
If you want to make your own chassis and put a loco type body on top , Oatley Electronics sell 12v and 24v motors(120-400watt motors!), gears and chains, along with speed and direction controllers for this type of use. What you are after is basically a small ‘tractor train’ , so have a look at a few of the model engineering magazines. None will have exactly hwat you want, but there will be enough info and suppliers listed to at least show you what can be purchased to make what you want.