When travelling on a Melbourne tram, you might have noticed a little window of sand next to your seat. So what’s the story?
@yarratrams I've always wondered about those little windows of sand on trams. What is the sand for? Why is there a window to it?
— Jessica (@jessvanc) July 26, 2012
Steel wheels rolling on steel rails might be a low friction surface for moving along efficiently, but when it is time to stop, a tram needs all of the friction it can get – and dropping sand onto the track is the easiest way to achieve it.
The story of sand on trams starts at the depot, where trucks deliver clean sand into a tall tower.
Hoppers onboard each tram carry the sand, which are refilled when required.
The little windows allow the amount of sand inside each hopper to be checked.
Inside the driver’s cab, there is a button to drop sand.
When pushed, the pipes from the hoppers open, dropping sand onto the tracks just in front of the wheels.
As well as giving extra friction between the tram wheels and the rails, sand also serves as an electrical insulator – an unwanted attribute that results in electrical arcing when too much builds up on the tracks.
The sand also ends up in the flangways of the rails, making a mess.
To prevent this, Yarra Trams has a fleet of modified street sweepers to vacuum the sand back up again.
Unlike a normal street sweeper, the tram track cleaner has a special attachment to get the last bits of sand of the rail flanges.
Unfortunately the circle of sand isn’t a complete one, as the recovered grit can’t be reused in trams – specially filtered and dried sand is required, or the hoppers and pipes get clogged up.