Sydney trains are running up to five hours late. How long will delays last?

Sydney's train network will face further derailment this week as several unions undertake industrial action, throwing commute plans into chaos.

About 400 services were nixed across the city today with more than 1000 expected by the end of the day.

Commuters waited hours while others were forced to find alternative transport and, with the afternoon commute chaos looming, more delays are anticipated tomorrow.

Here is everything you need to know to stay ahead of the disruptions.

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What is the Sydney train status?

Only 16 per cent of the city's morning peak service ran on time or within five minutes of their timetabled time – meaning more than 80 per cent were late or cancelled.

There will be more than 1000 train services cancelled by the time the day is up, NSW Transport Minister Jo Haylen said.

The delays are expected to get progressively worse, with chaos all but ensured for the afternoon peak-time commute.

"This is the boa constrictor strangling our network... This is the cumulative impact of hundreds of industrial bans," Haylen said.

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The action is particularly affecting the T1 North Shore and Western, T2 Leppington and Inner West, T3 Liverpool and Inner West, T4 Eastern Suburbs and Illawarra, T8 Airport and South, Blue Mountains, South Coast and the Central Coast and Newcastle lines.

Flow-on effects are being felt network-wide.

Commuters are urged to avoid all non-essential travel and stay home if possible as some services are running up to five hours late.

Some replacement buses are being organised but commuters are being warned not to rely on services this afternoon — especially the Central Coast and Newcastle Line which is expected to be facing some of the worst delays.

Metro and light rail services are unaffected.

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What was the cause of the Sydney train delays?

Several unions are conducting industrial action.

The Rail, Tram and Bus Union (RTBU) imposed a 23km/h reduction in speeds on sections of track where trains can normally travel up to 80km/h.

Today's speed reduction is one of eight industrial actions the union has restarted.

The Combined Rail Unions also renewed 350 work bans.

According to the state government, the Combined Rails Union was asked to complete critical maintenance overnight in Bondi and Homebush but refused to do so.

As a result, massive delays were felt this morning as signallers were forced to conduct manual work to keep trains moving along.

Maintenance was also due at Gosford about noon today but also was not completed.

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Why are train drivers taking action?

The RTBU wants a 32 per cent pay rise over four years.

Haylen's offer on the negotiation table is 13 per cent over four years with an additional one per cent in superannuation and another one per cent via a mutual gains bargaining agreement — a total of 15 per cent.

That agreement would see NSW Trains and Sydney Trains merge into one entity.

RTBU NSW Branch Secretary Toby Warnes said there is "a lot of work that needs to be done in relation to what the offer entails".

The union is currently considering the offer. The government has asked them to reply by tomorrow.

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Will the Sydney trains be delayed tomorrow?

Most likely.

Sydney Trains chief executive Matt Longland said the disruptions could worsen for commuters tomorrow and Friday.

"The advice that we're receiving, particularly from the Electrical Trades Union, is that there will be no movement on their work bans this evening and that these sorts of delays that we've seen today will continue into the day tomorrow," he said.

"So we're asking everyone to plan ahead.

"If you do need to travel tomorrow, keep up-to-date with the latest information. Use the travel apps to look at other modes of public transport that might be suitable to ensure that you can complete your trip in the most efficient way as possible."

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How long are the Sydney train delays going to last?

It looks like the industrial action will continue through to Friday.

The RBTU is hoped to respond to the government's offer — which is less than half of what the union is demanding — tomorrow.

It comes after months of failed negotiations, that almost threatened to cancel the city's New Year's Eve fireworks.

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