'A' isn't always for 'apple'. The question is, was 'A' ever for 'Armoured Train' ? (Bear with me; this IS leading somewhere).
It might not be 'Armoured Train' but it offers just as much insight: a view of the long shadow cast by The Great War on society, long after 1918. I didn't even know what an Ambulance Train was, so looked it up. What I read was moving.
You'll find more here:
This Ladybird Series book was probably not printed before 1924, so at least 6 years after the end of World War One. Sometimes it's the tiny details of daily life that can offer a glimpse into the past: a past where the term 'Ambulance Train' was seen as such a familiar piece of vocabulary that it was suitable for a child's alphabet book.