David Hansen writes:

A dangerous impression. If there was no (electrical) danger from the
central conductor rail then it would not be mounted on insulators.

Insulators are a give-away that something is energised.

Does that necessarily follow? If the reason for using a 4-rail system
rather than a 3-rail with return via the running rails were to avoid the
problems such as electrolysis and interaction with signalling, would it
not be possible to do it by having the centre insulated rail at a
nominal ground potential but only bonding it to ground at the
substations?