A lot of railways sell tickets only on site or using weird self built solutions for selling via their website. There is a slightly more professionall online ticketing system called https://www.fahrkartendrucker.de/ which was developped with support from the "Verband deutscher Museums- und Touristikbahnen", the overarching organization of many of the german heritage railways. This works, but is seriously outdated and lacking a lot of features.
A modern ticketing system which is scalable, works with many payment provider, works on all devices and provides PoC capability would be helpful. https://pretix.eu/about/de/ covers a lot of the needed functionality, but would need to be extended / configured to allow seat reservations and multiple trains and ticket types (return tickets, single fare, …). This is something which could be used by all railways, but would also require a simple instruction how to install/host or a central hosting with configuration service, which could be commercial.