TOPIC:

Maybe an image would be more helpful > the tool I m using is circled

It has a specific behavior, so try to play/test it a bit to fully understand it

BelSon : as Aldoras said, it's the PolyLine track tool (link to the origin). Basically it's a rail building tool that doesn't forget where last piece of track was placed, making the possible connections to where you drag the mouse. Requires practice but after you get used to it... hard to play without xD
Anyway, as in other custom clients, it's a patch (code changed, link above), included on BTPro client, not available on Vanilla.