Why are there stones on MRT track?

Png Png is very fond of taking MRT. Everyday, he tells me he saw me taking MRT home. (His imagination). He said he wants to fetch me home.


Chen Chen is on a week of school holiday. So, my sister in law can go home straight away after fetching Png Png from childcare center.


To my surprise, my sister in law brought Png Png to take MRT to "fetch" me home today. They took MRT to Tampines to meet me.


While in MRT,
Png Png : Mummy, why are there stones on MRT track?

Me : I don't know. I will find out and let you know.


I google search and the answer is
Crushed stones on railway track is called Ballast. Ballast is used to
- facilitate drainage of water,
- distribute the load from the railroad ties,
- keep down vegetation that might interfere with the track structure
- hold the track in place as the trains roll by.

I wouldn't question this if not for Png Png. I learnt something because of him.