Most of the tunnels in both the LNER(NBR) and LMS(Caledonian) Glasgow suburban systems followed the line on the roads above and as a result there were few if any ventilation shafts directly above (or at all), indeed the LNER tunnels were closed for a couple of days prior to the introduction of the Blue Trains to allow the smoke to clear from the tunnels. On the main line, Queen Street Tunnel had frequent shafts, but given the gradient and frequency of trains it needed them even into diesel days to clear the acrid fumes .
The Caledonian had an elaborate system on the Argyle Street tunnel to clear smoke from the tunnel, some description and links here
https://www.crassoc.org.uk/forum/viewtopic.php?t=140 given all accounts of the Stygian atmosphere at Glasgow Central Low Level and Glasgow Cross Stations it was not that efficient!