I decided this round to see if I could do a thing, which is to cultivate only a single trainspotter.
I think Eva is gorgeous and I really don't mind seeing her smiling face all the time, so I began collecting all the trainspotters other than her. Of course, to complete sets I have needed to collect a couple instances of her bright eyes, but aside from that I have been able to get up to 9 copies on board right now (beginning Era 6 on an express server). This means that now I only get one new trainspotter at a time, making it really hard to complete a set, but that's the price to be paid I suppose!
Eva is rare and it has taken 6 weeks to get to nine; I suspect it could be done a lot faster if one were to choose a more common spotter. But then you'd have to deal with slow set collection times that much sooner (and of course once you get to 10 you don't get any new spotters).
I took a screenshot that shows 8 spotters in close vicinity; the ninth is by another city and too far to fit into frame.