Abilty to change filters on trian cars while train is being built.

Not a biggie, but you can not change the filters of the train cars after it has started construction, you can do it before, you can do it after, but not during.
Why is it favorable behavior— because if you forget to set the filters you now have to wait for the train to finish or catch it before it reaches it first station. On a very long train this can take considerable time.
Thanks!