For DMK, you really should just take the A1 or A2 bus from Mo Chit. However, aside from the intercity services that transpac mentioned there are actually around 10-12 SRT commuter services a day that run between Hualumphong and Rangsit but which aren't listed on english timetables. Cost to DMK is only 10 baht but you don't want to wait so long for one of those when the buses run every 10-15 mins.

Makkasan CAT was over built to be a city check in for the ARL but also to be a hub for a future eastern HSR line. The city check in was never going to work short term especially with the 5 years of delays in extending the ARL to Bang Sue and DMK. Now that the privately funded Eastern HSR line will take over the ARL it will go someway towards fulfilling the original aim of Makkasan CAT. The ARL will need to be shut down for a lengthy period to configure the line for HSR operations and also as the rail that was used was cheaper, substandard Chinese steel.

Another metro line, the MRT Light Blue line, was previously also planned to run via the Makkasan CAT. However, this line has been deferred and is not part of the M-MAP2 list of lines to be built by 2029. Perhaps, thereafter?