They are the circuit breakers. Our trains in Brisbane, AU run 25kV (Thameslink seems to be the same 25kV overhead, with a 750V third rail, but this will be the overhead section) and the circuit breakers trip during the entry/exit of a neutral section, causing an almighty bang as they kick out and back in. If you're sitting in the wrong carriage directly underneath them it's very loud.
For reference, 1kV arcs across about 1cm of air, so 25kV needs a full foot of space in air (obviously less with a dielectric, but you get the orders of magnitude).
Source: I design trains for a living, though on the mechanical side
For reference, 1kV arcs across about 1cm of air, so 25kV needs a full foot of space in air (obviously less with a dielectric, but you get the orders of magnitude).
Source: I design trains for a living, though on the mechanical side