Their fees are outrageous
Rail Ninja is spamming Trust Pilot with 5 Star reviews in an attempt to get their ratings higher. I used Rail Ninja in Greece about a month ago. The website was quite good and I had no problem with booking the ticket I wanted. The issue is with their upcharge; it's ridiculously high. It was over 50% on my ticket. Rail Ninja charged $91 on what was a $55 ticket. (I didn't notice that I could have bought the $55 ticket directly from the Greek railway until after I'd already bought it from Rail Ninja). I've deleted their app and won't use them again.
