Commuters have been battling to make their way home amid packed carriages and overcrowded platforms, after a day of chaos on Sydney's rail network.

Key points: Major disruptions are affecting T1, T2, T3, T8 and T9 lines, with overcrowding on platforms

Major disruptions are affecting T1, T2, T3, T8 and T9 lines, with overcrowding on platforms Commuters have been urged to head home earlier than usual and expect delays

Commuters have been urged to head home earlier than usual and expect delays The delays stem from a loose hatch on a train that blocked a line on Friday morning

Large crowds queued just to get onto platforms at Sydney's main train stations, as the city's train network grappled with cascading delays from Friday morning's meltdown.

Sydney Trains Chief Executive Howard Collins said a loose hatch on a train that blocked a line at Town Hall was to blame for the problem.

A track circuit failure at Central Station was also delaying the network's recovery, Mr Collins said.

Many commuters expressed frustration at the delays and called for Friday's fares to be refunded.

Loading

In a tweet, Sydney Trains said it would assess requests for compensation "on a case-by-case basis".

It also warned commuters that trains were not running to timetable and could be making extra stops.

Commuters were advised to board the first train towards their destination, even if it meant changing trains later, due to changed stopping patterns.

Major disruptions continued into the night to services on the T1, T2, T3, T8 and T9 lines.

Earlier, Mr Collins urged commuters to leave for home sooner than they normally would.

"Just bear with us, but if you need to travel home tonight — if you've got an urgent appointment or you need to see your kids, look at some of the alternatives," he said.

"We're doing our best, but a big railway like this, with all the interconnections, may take some time."

Staff at Town Hall limited passengers' access to the platforms to avoid overcrowding, while those trying to enter Central Station were forced to queue in single-file lines.

At Wynyard, commuters struggled to board train carriages that were already full.

One commuter said a large crowd was waiting at Erskineville station for half an hour before staff thought to advise them that no trains were stopping there.

Another passenger, Sammii, said during her trip from Mascot to Macarthur she was crammed into a rail carriage after waiting for a train that was delayed for 45 minutes.

She said there was poor communication, and Sydney Trains should compensate commuters by providing a day of free travel.

"I finally got on a train and literally I was hanging on to another person so I wouldn't fall out of the carriage.

"We were just crammed in, it was horrible … there were just people literally sitting on top of each other, standing on top of each other. It was bad."

Another passenger said after a full day of delays commuters seemed "confused and exhausted".

"I had to wait for multiple trains to leave because they were too packed to board," he said.

"I'm trying to get to Penrith but as far as I can tell they've cancelled every train to Penrith.

"There's a kid in my carriage crying that she needs to go to the bathroom but her and her mother are stuck on the train."

Commuters cram onto a train at Wynyard. ( Twitter: @rx4kid )

Other commuters vented their frustration on social media, with some complaining of trains "randomly switching routes once people have boarded" and "no information on boards".

Others questioned how one troubled train could cause such a delay and have a huge knock-on effect.

Replacement buses in traffic gridlock

In the morning, people around the city were told to get on buses to complete their journey, but the situation on them was not much better.

Queues for replacement bus services were seen extending for several blocks at many stations including Wynyard, Newtown and Rhodes.

Central Station was crowded as commuters tried to avoid the afternoon peak. ( ABC News: Kevin Nguyen )

A gridlock with dozens of buses on the city side of the Harbour Bridge had only added to the chaos.

One commuter complained on social media they had been told to walk over the Harbour Bridge from Wynyard.

Repairs were completed at 9.00am.

Commuter Mick Bock got off a train at Newtown after it did not move for almost an hour.

A woman falls while people cram onto a train during peak hour mayhem. ( Twitter: EMI )

"My trip from Lewisham to Central is usually 15 minutes but after 45 minutes the driver basically told us to get off at Newtown, as the delay was only getting worse," he said.

"All the information I had said that there was worse to come.

"If I can be honest, my boss is a good guy and is understanding, as this happens all the time."

Platforms were packed with very little room to move and one passenger had fallen at North Strathfield Station while trying to board a full train.

Uber's prices had surged, with one commuter telling the ABC a 2.6-kilometre journey was priced to $66.

Outraged commuters demanded refunds on their trip.

"One train needs mechanical repairs and apparently shuts down the whole Sydney train system causing delays. How come I'm paying almost $10 a day for opal again?" wrote one Twitter user.

Mr Collins apologised for the inconvenience and admitted the system was "ancient".

He said because the loose hatch was close to overhead wires, power had to be isolated while crews conducted repairs.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 55 seconds 55 s Replacement buses have choked roads through Sydney's CBD.

"Because we've got a pretty ancient system, we have to send people down into tunnels to pull old-fashioned switches to turn the power off just at this platform," he said.

Mr Collins said a "huge investigation" would be undertaken as hatches rarely become loose due to attached safety catches, but the problem appeared to be "external".

"As everyone knows who was in the city last night, it was pretty windy, it could've been a branch or a rock," he said.

A report commissioned after Sydney's rail network ground to a halt twice last summer found the city's "tangled" rail network and crew shortages were major factors in those debacles.