MTH makes conversion kits for this purpose, and they work very well. I had my PS1 Premier Blue Comet converted to PS3 about a year ago and am very pleased with the results. If you have a PS1 loco that you like and want to keep, having it converted to PS3 is a great alternative to a complete replacement. The conversion kits can be used to upgrade other brands as well. I just had a Williams loco converted using the MTH conversion kit and have a second one on the way.
John