As the Safety & Accreditation Manager for my group, I am familiar with the requirements of Rail Safety Critical Worker (SCW) medicals.
Firstly, your organisation should have defined the risk categories for the various operational roles it has. This would have been done by undertaking a task Risk Assessment, to classify the roles as to whether they rate an SCW Cat. 1, Cat. 2 or Cat. 3.
Your railway would then issue you with a Health Assessment Request and Report Form which would specify which SCW Level is required. It is unlikely a SCW Cat. 1 would be required unless you were going to be rostered on Driver Only (D.O.O.) trains or doing mainline work. Most preserved railway driving / firing jobs require a Cat. 2 at most. However, you may elect to get a Cat. 1 if you have plans to (maybe) apply for a full time professional role with a commercial operator on the mainline. However, it is more expensive (longer and more involved appointments and tests) and unless you are willing to pay for it yourself (cannot claim it on Medicare), your preserved railway is unlikely to fund it, if it is not required.
You will also receive a Health Questionnaire form to complete and an SCW Health Assessment Record for a Health Professional (for your doctor to complete). You will need to take all forms plus available workplace medical records (such as recent audiometry tests and last optometry results) and any information relating to recent illnesses or injuries (esp. workplace related).
Your railway should have a designated doctor (authorised health professional) who is familiar with the National Standard for SCW Health Assessments and is approved by the National Transport Commission (NTC) to undertake railway SCW Health Assessments. Unless your local G.P. is familiar with the standard, he cannot do the tests for you. If your railway doesn't have a designated doctor, you may be able to find one your local mainline commercial rail operator uses. However, it is best to have one most operators on your preserved railway uses, as he/she would then be familiar with the requirements of your organisation.
The outcomes of your Health Assessment will determine which roles you can then undertake with your organisation. This will be determined by the person responsible for Safety & Accredition with your organisation.
Health Assessments need to be redone every five years up to age 50, then every 2 years up to age 60, then yearly thereafter, unless you suffer a major illness or injury in the meantime, when a "triggered" health assessment will be required before you can resume duties.
Finally, the issue pertaining to eye-sight fitness is principally colour perception. You need to be able to pass the Isahara test (flash cards with coloured dots forming shapes in a background of contrasting coloured dots). Obviously essential for determining signal aspects! Myopia (short-sightedness) is acceptable with corrective glasses, as long as the glasses give you acceptable vision (you will be tested with your glasses on).
I hope this info helps.