This is posted not as advice, but as a way of guiding you towards going to the HSE website and consulting with the Office of the Rail Regulator in the process of determining which is your best solution, refurbishment or replacement.
Memorandum of guidance on the Electricity at Work Regulations 1989
"Electric railway and tramway operators, in conjunction with the Office of the Rail Regulator
(HM Railway Inspectorate), have developed standards and safety specifications for the construction of those parts of their systems which use bare conductors at overhead and at track level, together with safe systems of work."
Even working at the edges of the rail industry like me you get to realise that it is a highly regulated industry and that there are specifications and standards written for most products used on the railways and for most actions undertaken on the railways.
If there aren't any specifications and standards written to meet your current needs, you are going to have to design and write them, and if necessary get them agreed with the rail regulator. It may help if you buy in the time of a consulant experienced with this old equipment to help you author the standards and specifications.
If the original manufacturer worked to a set of standards in terms of component quality control and test specifications, that their equipment had to meet prior to being installed on the railway, then I expect you will have to at least follow these in refurbishing the equipment. If the regulator has released more modern standards they may want you to follow these instead.
If you decide to refurbish your own equipment, you will have to document the whole process (including changes in design or component specification) as any original manufacturer would, so that you can prove on inspection that the new design (and your implementation of it) meets the necessary standards set by the regulator and also any in-house standards and working procedures you have separately set down in writing as a company, as part of your quality system for example; e.g. in terms of safety, quality control, manufacturing specification, test specification, installation, maintenance procedures and inspection documentation etc. etc
(In terms of the corrosion you are experiencing this may be just down to old age, or being newly amplified by DC leakage currents or chemical potentials set up by the wrong metals in contact with each other.)