I used to buy jb-kl bed ticket with RM at jb then just took SGD $2 train from Tanjung Pagar to jb. Iirc the ticket price is same but one side use sgd while the other use RM.

The plus side was you clear both immigration check at Tanjung pagar at one go. So don't need to get down the train at JB.

I am not too sure how the custom works at JB train station now.