Autism and Autism Spectrum Disorders (ASD) includes a huge range of symptoms some of which remain for life while others recede at the onset of adulthood. Every individual is unique and displays different symptoms and every one goes on to a different future. The majority of milder cases will go on to live otherwise normal lives; some with severe autism will require care 24-7 for all their days.
As others have said there is no obstacle presented to train driving by a diagnosis of autism as such. It is the many tests and assessments and the ability to learn in a "normal" classroom environment and do the job safely that matter. No-one can tell for sure how a 6 year old will perform in 12 years or so when the time cones to consider paid employment. ASD apparent today might be gone by then. Or there may be some lingering insularity and dependence on others. It's way too soon to say. What needs to be borne in mind though is that if the recruitment process is passed there is unlikely to be any special consideration given during the very arduous year-plus of on-job learning. It can be hard and it relies on a good deal of independent study as well. Classrooms can be unforgiving places for ASD people.
Another factor to consider is the degree to which any ASD person can cope with independent decision making. Many are superb; some cannot make decisions at all. A train driver must make appropriate safety-critical decisions at every moment of every day with no margin for error or self-doubt. And they must be capable of taking control in any emergency and leading the initial response.
I can comment with some degree of authority. I have three nephews who all endured ASD and required special schooling or special teaching in mainstream schools. Two are now out in the workforce while the third has not yet developed appreciable social skills and while he is working it is in a controlled and non-challenging environment. Only one has been able to learn to drive a car. All have improved significantly from the childhood condition as they entered their twenties and one would probably pass as "normal" now for many purposes.
May I wish you every success and strength in the years ahead and with your daughter's career aspirations. She should seize every opportunity and work towards the goal she herself has set.