According to this Modern Railways article on the GA FLIRTs, the sliding step uses an ultrasonic sensor. It mentions that the door drives and step access systems are supplied by IFE, Knorr-Bremse's door systems subsidiary. Specifically, it looks like the Class 777s are fitted with a version of their X4 Sliding Step.What kind of sensor are we talking about?
That article actually mentions a possible failure condition that potentially seems to be similar to the issue is that's being seen with heavily-loaded 777s:
"One scenario to avoid is if the step comes out and gets under the platform edge, then the vehicle unloads and rises."
Thanks to Indian Railways, there is a PDF copy of the manual for IFE's Door Control Module (DCM) available (it's relating to a single leaf plug door installtion, but I expect the underlying mechanics and control scheme are similar, if not the same). The Step Module has it's own control module that supplies 3 inputs into the DCM - 'Step Closed', 'Step Brake Active' and 'Step Emergency Device Deployed' (this it to indicate if emergency manual deployment of the step has taken place).
The 'Step Closed' signal is used by the DCM as basically the last check in it's 'Door Closing' logic. Once the step has retracted, the signal is sent to the DCM which indicates the 'Door Closing' sequence has successfully completed and the DCM outputs a 'Door Closed & Locked' signal to the train's control system.
The 'Step Brake Active' signal is used in the 'Door Opening' logic. Once the Step Module detects it has deployed the step to within it's preset limit distance to the platform, it activates an electric brake on the jackscrew and outputs the signal. Once the DCM receives the signal that the brake is applied, it can can then start the rest of the 'Door Opening' sequence.
Table 4.3 on page 45 of the PDF describes logic controlling the 'Door Safety Loop' output to the train's control system. If the step is not closed, the train can't move (unless the individual step is electrically isolated from the cab, if the 777s have that option).
Section 5.7.2 on page 55 of the PDF describes the obstacle detection system for the Step Module.
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It will just be a pain for passengers on those lines though for a few weeks which is unfortunate