I am currently running another RTR loco in, this time a Class 24 in P4, which is destined for use on 'Callow Lane'.
Although I do use a rolling road, I prefer, where possible, to undertake running in on actual track.
The P4 circuit is 7' 6" in diameter and is effectively a circle of 'P4 set track', made up from sections of C&L flexi track, curved and held in the correct radius by copper clad sleepers.
I wish I had a permanent space for this, but the dining