[David_Hayward (RIP)]

That's pretty good going then! As i said you can imagine that a partilcular engine is placed down at the front of a huge are of similar units, and then taken away and railed off, and then chosen again, so that the factors align.

On the other hand there were serious out-of-sequence numbers where engines presumably stuck at the back, or perhaps had a problem than needed rectifying, and so were used much later. However it gives us some idea of dating engines by their number. Thanks very much for the input; more is needed!