The original Tri-ang product used the tools of its time - and in the case of speed controllers this meant rheostats made of wound copper wire to reduce the 40 Volt DC to whatever voltage was required for slow running.
Terribly inefficient, as the power that was not 'used' by the motor was wasted as heat.
But it was 'low tech', low cost - and worked.
The 'modern' solution to motor speed control is the same as used for all electric trains, from Z gauge to standard gauge and that is to 'chop' the DC by means of electronics to reduce the power available to the motor.
Very efficient, but not cheap and prone to sulking when shorted out, connected back to front and similar trials and tribulations.
If one adds regenerative braking to the system them the inertia of the train is used to recharge the batteries - all good stuff!
If you are brave and know what you are doing, then you could design and build a suitable controller or you could do what we did and buy one ready made.
There are a few UK based suppliers, with two 'market leaders' in Parkside Electronics and 4QD.
Both supply similar products at similar prices - we first used Parkside as they make a controller which can be used with the original Tri-ang /GEC motor but have also used them with our 'upgraded' Bosch powered locos. £200 or less will buy you a 100 amp, reversing controller with regenerative braking.
We have also experimented with controller made in the PRC which can be found on E-bay for £20 or so. They seem to work and are used on the Scamp loco of which over 100 have been built.