Apols but I didn't see this until now. Not really a future hub for BKK but a hub for the east yes. The new Bang Sue station will essentially replace Hualumphong station and be the main hub for intercity trains.

The Makkasan Railyards are a huge area of SRT land in the inner city, much of it unused. Around 12/13 years ago, it was proposed to redevelop the railyards and build a mini inner city here, office building, shopping malls, condos. Part of that initial concept plan was to have 4 100+ story buildings! And very little open space.

When the ARL was belatedly approved (after it was previously deferred for 2 years), the obvious and only feasible option was to build the City Air Terminal (CAT) station on the Makkasan railyards. The CAT was built the size it is both to cater for future pax expansion but mainly due that it would be the main station for this inner city redevelopment. There was also a potential it would become the Terminus station for the Eastern Lines - which is the current Rayong HSR line plan. The consultants for the ARL did forecast incorrectly that many more people would use the Express service.

The CAT will have a future Black metro line pass and a planned monorail around the Makkasan redevelopment to serve the area would terminate link the CAT.

The Makkasan redevelopment has been reworked a number of times and there is a large community campaign to ensure that there is at least 30% open space/park. This is the current concept plan;

The CAT is at the far right.

As such the CAT is a suitable size for future plans in this area and will over the next 20 years become more of an important transport interchange.