Why is it necessary to convert 2 rail rolling stock to 3 rail operation?
A number of reasons, but mainly cost. Original three rail powered units are either old or very expensive or a combination of both.
There is new three rail stock available but this is all specialist continental stuff, and is expensive. Marklin is probably the biggest producer. Three rail locos can be acquired for moderate cost but these are usually old Hornby Dublo units which while generally very tough show thier age. Also there is a limited choice.
To run a new locomotive on old track, probably with old rolling stock requires the locomotive to be converted.
The Conversion process.
This requires two things. First of all the loco has to be rewired and secondly some form of central collector added.
Rewiring is generally straight forward.
First of all access to the supply tags on the motor are required, or the the DCC card input. DCC is practical with three rail.
On a two rail system one input is from one rail the other from the other rail. So all that is required is the wire to one terminal to be unsoldered and then soldered to the other terminal so that one input is from both running rails. A central collector now needs to be added to provide the supply to the now disconnected terminal
This is where the fun starts.
Orignal three rail locos are designed to accomodate a collector. Early Hornby Dublo two rail locos were often based on three rail models and can have suitable attachment points available. Most though are new and really only general rules can be applied.
Practicalities are detailed on the following pages.