Fears of overcrowding at train stations due to a shutdown of the Opal system have been realised, with one of the first known instances of a glitch affecting commuters during peak hour in the Sydney CBD.

On Wednesday, Fairfax Media reported that Sydney Trains staff were being told to close down ticket gates for about four minutes when Opal card readers malfunctioned, which prompted alarm among some staff in internal emails about overcrowding in peak periods.

What Town Hall looked like Thursday morning. Credit:SMH reader Sheridan Ah Colt

On Thursday, a commuter who witnessed a reboot of barrier gates just before 9am at Town Hall station said the procedure resulted in a five-minute delay in getting out and chaos among passengers, who couldn't move.

Have you witnessed shutdowns during peak hour? Email us with photos