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If you want to go Slavic, Sergei Lukyanenko's Night Watch series is very digestible SF/Horror/Fantasy. Even has a vampire in space at one point. Slavic, so not terribly cheery but does have a sense of humour. Has a protagonist who becomes a major player entirely by fluke and not due to any inherent qualities.
Staying Slavic and going post-apocalyptic SF, there's Dmitry Glukhovsky's Metro series (whence the video game franchise). Relentlessly grim, but gripping and rolls along at pace.
Still Slavic, the first two books of the Witcher series are short story collections with lightweight bridging arcs. The novels that follow are probably a bit much for interrail.
Any recommendations for decent-length but fairly digestible sci-fi and fantasy series? Interrailing over the next couple of weeks. Something along the lines of James S A Corey, Alastair Reynolds, Naomi Novik, N K Jemisin, etc