"Hearing and speaking requirements (3) Sufficient hearing confirmed by an audiogram, i.e.—
– hearing good enough to hold a phone conversation and to be able to hear warning sounds and radio messages.
The following values should be taken as guidelines—
(a) the hearing deficiency must not be higher than 40 dB at 500 and 1000 Hz, (b) the hearing deficiency must not be higher than 45 dB at 2000 Hz for the ear with the worst air conduction of sound, (c) no anomaly of the vestibular system, (d) no chronic speech disorder (given the necessity to exchange messages loudly and clearly),
(e) the use of hearing aids is allowed in special cases.
Infrastructure managers and railway undertakings should consider whether some occupations, even those where mandatory hearing standards currently exist, require additional or more stringent hearing standards in order to control other identified risks. This might be, for example, a train driver working on or near the line. It should be noted that the widely adopted hearing standard for persons who are required to maintain their own safety when working on or near the line is that hearing loss should not exceed 30 dB averaged over frequencies 0.5, 1.0 and 2.0 kHz in either ear."
I did used to work with a Driver with a hearing aid at my previous company.