It's traditionally and still a very DACH dominated event, for better or worse. Travel by regional/interregional trains around this time is perfectly fine, and it's a time where it's quite easy for a whole big community to synchronize for taking vacation.
Remember that this is not the type of Congress where people go on paid time. There are no sponsorships and generally no commercial interest behind this event, so I think (my guess, I have no data to back that up) that most people take vacation days and pay their own ticket.
("DACH" meaning Deutschland, Austria, Chwitzerland, if I'm not mistaken; i.e. the main German-speaking region. Not sure this is common knowledge outside of people that are in / regularly deal with DACH; I only learned of it 2 years after moving to Germany, being from the Netherlands.)
It’s a good chance to use your Bildungsurlaub allowance (education vacation?) where for 5 days a year you are being paid by your employer for educating yourself in a way of your choosing. C3 is recognized for that program almost everywhere in Germany.
Remember that this is not the type of Congress where people go on paid time. There are no sponsorships and generally no commercial interest behind this event, so I think (my guess, I have no data to back that up) that most people take vacation days and pay their own ticket.