What is the initial action required when closing up to a stalled train in a station under CBTC?

The initial action required when closing up to a stalled train in a station under Communications-Based Train Control (CBTC) is to proceed in normal operating mode until the Maintenance Action Level (MAL) permits transition. This approach aligns with the operational protocols designed to ensure the safety and efficiency of train movements.

When operating in CBTC, the system incorporates various levels of operational control that guide train engineers in responding to different scenarios, including stalled trains. By maintaining normal operating mode initially, the engineer can remain within the established parameters and ensure that they are following the system's protocols. This also allows for proper assessment of the situation while ensuring that the train can safely approach the stalled train without causing additional hazards.

Other options involve transitioning to different modes or taking actions that might be premature or unnecessary at that stage. Immediate transitions or emergency braking could result in unsafe conditions or further complications if the situation does not require such drastic actions. By first operating in the normal mode, train engineers can effectively manage the situation while awaiting further instructions from the control system or maintenance protocols.