Changing Employee position

Does changing employee position decrease effectiveness or have any setbacks?

I know there is some effect of an employee 'getting used to the company' in the first few days of them being hired, but what about change of positions?

Specifically: employee is in position A, switch to positon B for Trains, switch back to position A immediately.

Is that bad / has setback or negative consequence? Is it ok to do daily or should it be minimized?

I've moved employees around a fair amount, including temporary "just to train them" moves like you're talking about. I don't see any sign that it does any harm. I do think that you don't get their full effectiveness for 2-3 days after they join, but moving them from one job to another doesn't seem to reset that.