At the same time on Collins Street, just before Queen Street, a tram driver told passengers the ride would be slow for the next little while. That's because, the driver said, the tracks were slippery. It wasn't just due to rain.

It was because leaves had become stuck on the tracks; pressed onto the steel by the weight of the passing trams, but made more adhesive by the rain. The driver pushed on, moving slowly, only a few metres per minute.

By the time the tram stopped near William Street, many hopped off, while others remained for the tedious but necessarily careful journey toward Spencer Street. A passenger told Fairfax Media at no point did he feel the tram was in danger of slipping off the tracks, but that the driver explained this was a risk if the tram did not continue slowly.

The driver then said, according to the passenger, that a Yarra Trams' cleaning truck was on its way to scrape the leaves from the tracks.

This is why on Saturday a tram driver on Collins Street said autumn was the hardest, or at least the most slippery, time to drive a tram. She said that the tram tracks became greasy, like footpaths did, when wet leaves had been pressed into them.

To manage driving on the slippery tracks, she said she could either drive slowly (Yarra Trams also trains drivers to adjust braking distances in the wet) or press a button on her dashboard labelled "sand". This dropped sand onto the tracks and stopped the tram from "skidding".

Despite those safety measures, Rail Tram and Bus Union national president Phil Altieri said autumn tram driving was still too dangerous.

"Autumn is the worst time for our system and the most unsafe time for tram drivers," Mr Altieri said. "The leaves have lots of oily secretion in them, they leave a greasy residue on the tracks that turns into black soot over time.

"It makes the track very slippery ??? it has been going on for donkey's years but what is more concerning is the way Yarra Trams deals with it."

A Yarra Trams spokesperson defended its track-cleaning regime.

"Safety is our priority and a long-established track cleaning regime ensures tram tracks are in the right condition to enable safe tram trips," the spokesperson said. "Yarra Trams is doing more track cleaning than ever before."

But Mr Altieri said Yarra Trams had outsourced track cleaning to a firm called ISS, which didn't do enough preventative or reactionary track cleaning in autumn. He said the leaves and their "oily residue" can cause trams to skid, particularly when travelling on an incline or decline.

Mr Altieri said autumn was the worst time of the year for slippery leaves on tracks, but that dirty tracks at other times of the year could cause electric shock. This can occur when a tram becomes "isolated", or when a tram is on but its wheels are not in direct contact with the tracks.

Yarra Trams said electric shock very rarely occurred to anyone using the tram network and that areas where more leaves fall are cleaned more regularly.