Refactor merge train position

What does this MR do and why?

• Refactors how the client accesses the position of a car in a merge train

I'll need to create another MR to look into mergeTrainCount, unsure atm if we can refactor this everywhere to use mergeTrains field.

MR acceptance checklist

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Index

How to set up and validate locally

1. Add a MR to a train
2. Verify merge train position is working as expected

Related to #475269 (closed)