Thanks for the examples
Why does the driver of a rescuing unit need to access (via nose end door) the unit being rescued ? To rescue a 717 in the Norther City Line the driver of the rescuing unit doesn't need access to the failed unit. Unless the push out is of a train that was dead and uncrewed ?
In theory they don’t. However in practice there may well be other staff who will want or need to cross from one train to the other.
You might have a train technician or emergency response staff, for example, even if it’s just to do a walkthrough and check on the welfare of passengers who might have been stuck for a while in a hot tunnel.
It’s worth mentioning that coupling Underground trains together for rescue purposes is exceptionally rare. For something like the Northern Line fleet it will probably have only ever happened in anger a couple of times, certainly since the stock has matured after initial teething issues.
If a train has broken down, it’s more likely response staff will be sent to the train (either by walking down the tunnel or via another train), and in most cases will find a way to get it moving. The rolling stock training provided to drivers has been slimmed down over time, and unfortunately this can manifest itself in unfamiliarity when something does go wrong - especially on the fleets like the 92/96/09 stocks which basically almost never break down or give anything but minor problems.