The line to central Singapore has been closed, but there is still a pathetic shuttle service that crosses the causeway from Johor Bahru:

https://en.wikipedia.org/wiki/KTM_Intercity#Operational_serv...

There are plans to replace this with a metro service and eventually build high-speed rail between Kuala Lumpur and Singapore, but both have been repeatedly bogged down by politics.

https://en.wikipedia.org/wiki/Johor_Bahru%E2%80%93Singapore_...

https://en.wikipedia.org/wiki/Kuala_Lumpur%E2%80%93Singapore...