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On the North London line 416’s replaces the 501,s prior to 313 appearing. Did the EPB’s ever work the Euston Watford service or did that go straight from 501’s to 313’s?
Yes, that's my recollection too. I can't find anything to confirm this, but I think that the 501s might have been fitted with tripcocks for use on the section shared with the underground. The EPBs definitely didn't have them, so that could be the reason why they could not be used other than on the North London Line.