Which is the most recommended sensor for sensing humans inside a rail passenger car?
Doppler RADAR can be used to provide an excellent person sensor, perhaps in conjunction with other secondary sensors. Doppler RADAR modules are available at low cost.   

Secondary sensing could be by eg   

PIR - motion detection.
IR - body heat
Ultrasonic  - motion detection
IR - beam breaking
Capacitive - body proximity. 

Doppler RADAR application:

Person movement along a carriage can be detected by horizontal units and individual seats or areas by vertical units.

As well as whole body motion it is possible to detect physiological body attributes


Knee joint articulation produces a frequency of about 25 Hz due to the action of the joint under load. I long ago met people who developed systems based on this feature. They claimed that it reliably detected people and rejected flapping curtains falling boxes and miscellaneous pussy cats. 
It should be possible to detect heart motion artefacts. This may require sensors located in seats. The cost of the RADAR units is low enough that a sensor per seat system may be economic.




The large range of low cost Doppler RADAR units appear to be based on the "RCWL-0516" module.

From here:  




" ... The electronics of the module consists of two equally important sections: a microwave frequency transmitter/receiver/mixer based on the MMBR941M high-frequency NPN transistor and a much-lower-frequency section based on an IC — RCWL-9196. Technically, the microwave section resembles a “Colpitt oscillator,” with the requisite inductor (and capacitors) made by circuit board traces. The inductor (~10 nH) is the S curve trace on the top surface, and capacitors are the ring structure on the bottom surface and also the rectangular block to the left of the S curve. ..."


Useful application article
and another 

This Google search provides many related links.

Many Ali Express Doppler RADAR units  - buyer beware as ever. 

Under $US1 each! 


Quality unknown.