The thing surely is that "modern image" is a phrase which is entirely likely to be used in ordinary conversation unrelated to railways. For me, in a model railways context it will make me think WCML blue electrics, because I was reading RM when they started using the phrase with that meaning. But without that experience, I would expect it to mean today's railway - and its railway meaning is therefore ambiguous. If the particular arty period had been labelled "New Art", the same objection would apply, but sticking in the French means "Art nouveau" has held its original specific meaning. Perhaps if RM had used the label "image moderne" instead ........
For French scholars - I'm sure there should be an accent somewhere in "moderne", but don't know where, which, or how to type them anyway .