How is the word "chloride" being used in the name of these lights? I don't understand its meaning outside of a chemistry context.
The handlamps are/were manufactured by the Chloride Bardic Ltd in the UK.
Originaly, decades ago, the main business of Chloride batteries Ltd, as the company was then known, was the manufacture of lead acid batteries.
They held a patent for an improved manufacturing process for lead acid batteries that involved the use of lead CHLORIDE.
The trading name "chlroride batteries" still exists but AFAIK it is now owned by some large conglomerate. Still known for batteries, especialy for older type electric vehicles and for industrial standby purposes.
In common parlance on the UK railways "bardic lamp" is often misused to refer to ANY battery operated portable lamp that can give light of different colours.
A fleabay search for "bardic lamp" will turn up numerous new and used genuine bardic lamps, and also numerous other colour changing lamps and flashlights of quite different designs.
Apart from use as a handheld lamp for various signalling purposes, the bardic handlamps can serve as a temporary headlight or tailight on a train in the event of failure of the built in lights. Trains are equiped with a bracket to hold either a purpose made tail lamp or a handlamp.
In the days when batteries for flashlights and cycle lamps were expensive, railway staff used to use the issued handlamps on bicycles, thereby avoiding buying batteries.