For scotrail would ardrossan town to harbour be shorter than central to Argyll st? Can't be much in it.
RTT shows Ardrossan Town to Harbour as 38 chains but Central to Argyle Street as 28 chains. Certainly close, however.
Merseyrail's two shortest runs are around 25 chains in the form of James Street to Moorfields (24.8 to be precise, if my maths is correct) and Wallasey Village to Wallasey Grove Road which shows as 25 chains on RTT. Honourable mention goes to Walton to Orrell Park at 30 chains.