Comparing year-on-year Transport for NSW data for the period of March to November (December figures are not yet available) show rail trips on the South Coast line in 2020 plummeted by 60 per cent compared to last year.

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In 2019, more than 31 million trips were taken on the South Coast line through the period of March to November, compared to just 12,499,023 in 2020.

That's a drop of more than 19 million trips - an average of more than 2 million lost trips each month.

The biggest month-on-month falls happened in the early months of the pandemic.

The data for April shows a whopping 84 per cent decline, from just over 3 million to just 502,883.

That's an even bigger drop than what occurred in the following month, which saw the introduction of social distancing measures like green dots to point out which seats commuters could safely sit on.

The measures saw the recommended capacity for trains fall 32 people per carriage - or 256 on an eight-car train.

An eight-car train can normally seat around 900 people, which meant more than 600 missed out.

In May 2020, the number of South Coast train trips taken actually rose compared to the previous month to 849,627 - but that figure was still 79 per cent down on the same month last year.

A Transport for NSW spokeswoman said it was not possible to state how much revenue the South Coast line had lost due to those missing 19 million trips - but a figure for the shortfall across all the rail lines was available.

"The COVID-19 situation's impact on farebox revenue for the 2019-20 financial year was $361 million," the Transport for NSW spokeswoman said.

"Additional temporary transport services in Greater Sydney to help spread the peak and help allow customers to physically distance on services are currently costing approximately $530,000 per week."

Those extra services were introduced in June but none of them made their way to the South Coast line due to it being "constrained in the number of trains already on the line", a spokesman said at the time.

The Transport for NSW spokewoman said the farebox revenue did not "cover the cost of operational expenditure requirements for the delivery of public transport services".

Due to Transport for NSW taking the advice of NSW Health when it came to COVID-safe practices, the spokeswoman said they were unable to state when passenger numbers would return to normal.

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