Oh yeah that's a little rare but it does happen from time to time. The layout at Darlington has always been very flexible (apart from that one little niggle). It was more painful when a down LNER (or XC/TPE) had to pass a Northern at the southern end of P4. The bypass loop is painfully slow compared to the direct P4 approach (10/20mph I think versus maybe 50 or 60?).
All the more time for northbound passengers to savour the sight of the magnificent train shed !
== Doublepost prevention - post automatically merged: ==
But extremely difficult to replace the switched diamond with 3 turnouts in the space available to keep the connection to Eagleascliffe. Could possibly be done though - but may require losing the line adjacent to 1 and moving the turnouts for 2/3 on to the Down Loop.
I suspect the opportunity was identified in the project, but presumably passed up for cost reasons.
Bypass loop is 20mph, southern half of 4 is 35mph according to the Sectional appendix. It changes to 50mph in the northern half after the points from the loop.
Don't think I've ever seen anything on the line next to the P1 line.