A Hardware Accerlerator is a little piece of chipboard that contains some computer chips to improve the way older and even newer PC`s Handle games. They add more power to a PC, much like a turbo in a car.
You should be able to pick up a cheap Geforce 2 at your local PC shop for around 20 to 30 pounds, this should be ok for your grandsons train simulator.
Hope this helps
if you want more detailed information e-mail me the system specs of your gransons pc and I`ll help if I can