It is as simple as a resistor in the ground side of r2 to get 4v min, try a 4k7, it depends on your trans voltage so you'll need to experiment, a 5v1 zener might be a better idea if your powering other motors from the same transformer to give a bit of stability, cathode to R2.
A Tip122 is a 5a/100v readily available darlington, zrx450's are still available and 2n3055's are still common (although some electronic hobbyists will tell you they are old), a more modern version of it is a Tip3055 which is the same tranny in a more modern package.
To power electronic accesories within the loco's you'll probably need a 1n4001 series diode and a 1000u capacitor, especially if these gizmo's contain a microcontroller, as with pulsed dc they'd get all upset. it would make sense that these devices would already be installed on such accessories so have alook first.
Hows the weather in Plymouth?