Current excess capacity is zero without some investment. 20Mtpa...

Current excess capacity is zero without some investment. 20Mtpa is very possible with some investment by adding sidings and maybe by twinning the last 20 odd kms. 100Mtpa is very possible but is will require relaying some track, and laying extensive sidings. It would require additional berths in the port. 300Mtpa plus is possible by twinning all lines. However this latter volume means a serious port upgrade. This is the simplest way of getting the product to port.
There are four other suitable means of getting the product to a ship. One needs transhipment to a deeper loading area and will need new rail. Another is to a suitable bay for a deep water port but needs both the port and rail infrastructure. The other two are existing ports but will need additional infrastructure costs.

20Mtpa works now, and will work while demand grows, but long term 100 Mtpa works best.