I think this was discussed here not that long ago and from memory, people preferred the 'ise' endings of most words.
Do we really think that people's responses to the railways are so knife-edge that using an Americanised word ending will tip the balance against them?
I'd respectfully suggest that the railway are looking for
- a response at all
- writing which is clear enough that the reader (ie. the railway) will know what is meant, and
- a degree of demonstration by the writer that they have done wrong and will not do it again
Everything else (and everything includes punctuation and spelling beyond what is essential for meeting the above points) is an optional extra and it seems to me that we won't be helping posters here if we worry about any of the everything else.