Electronics 2000

I've read tonnes of stuff on this but still struggling.
I'm building my own model train controllers as a hand held unit. Decided that PWM was the way to go and found a cheap ready made unit which I purchased. It works but not exactly as required when fed from a fixed 12volt supply.
The trains involved are a mix of new and old all 12volt dc and drawing .25amp(new ones) upto 1amp(old ones). The problem is that the old trains work fine in terms of start speed and top speed but the new ones are way to fast - dangerously so even from start. The PWM is a 3amp 6volt to 15volt unit. I've done some tests with an old variable transformer feeding the PWM and when it is turned down alittle it will allow very good control of the new locos through the PWM.
So what I recon I need is a variable input voltage for the PWM. I have a 10K lin pot 0.4w but don't know how to go about creating a small (handheld remember) unit to install infront of the PWM. I am capable of building the units but design is not my strong point.
Any Ideas.
Thanks for reading.
Brian

Best solution would be Programmable logical controller. I would choose Microchip PIC16F84A microcontroller. You can build your own programmer for that, but you need RS323 serial connection on your computer, of course you can build USB programmer, but it is more complex, because you would need serial to USB converter, that is one separate chip.
PIC is cheap,
PIC is made for single task at a time, can't do parallel functions at the same time.
Its slow, but enough for controlling trains, it can generate PWM signal, you can program several PWM generation modes, for different pins, which could be speed buttons, if you want not shift like speed regulator, then you need only transformer and varistor, with which you can control the output current, higher current, higher rpm.