Not sure how short a line counts for this thread, but a couple in the Newton-le-Willows area (which form part of longer routes) are:-
- Parkside Junction to Lowton Junction was last used for regular scheduled passenger trains when the Manchester Exchange/Glasgow sleepers finished around 1969.
The Parkside East chord stayed open for goods, parcels, light engines etc. until TransPennine Express started their Manchester Airport - Glasgow/Edinburgh passenger services around 2007. These were DMU-operated at first (185s), until the eastern half of the Chat Moss line was electrified at the end of 2013 and TPE Class 350 EMUs took over the Glasgow/Edinburgh route during 2014.
The TPE services have gone over to the Bolton route, but Northern still operates Class 331 EMUs this way several times during peak hours (Wigan NW to Manchester Vic)
- Earlestown South Junction to Earlestown West Junction (via Platform 3) used to have regular passenger trains until the St.Helens Shaw St - St.Helens Jn - Warrington B.Q. railmotor ended in 1964 as a result of Beeching.
There was then a hiatus with no booked passenger trains until an hourly Lime Street - Warrington BQ. DMU was introduced by Merseytravel sometime in the 2000s. This became one of the 'Northern Electric' Class 319s after electrification of the Earlestown West curve in 2015, but I think gave up the ghost during COVID circa 2020. As I understand it there's only a 1 per day (if it runs) Ellesmere Port to Lime St DMU over the west curve these days. Was this one of the shortest-lived "reopened" electric services?
Another "could have been" example to the OP is the Hunt's Cross to Gateacre line.
When the residual Gateacre branch closed in the late1970s, I think the medium-term aspiration was to re-open it as an extension of the electrified Liverpool Central - Garston line. (some ambitious plans back in the day had Gateacre as an interim terminus on a fully re-opened Liverpool outer circle rail route). As things worked out. of course, the third rail only got as far as Hunts Cross, and Gateacre was a dead duck by that time.