On Sunday there is also not as much pressure to get things running as there would be on Monday. Or perhaps this is the incentive to pay now and get things fixed on time for the work week? In that case Friday night would again have made more sense unless the attackers have some very specific insight into how much slower restoring without paying is.

My alternative theory is an expired certificate that makes some core systems just not talk to each other anymore. The announcements on the stations, for example, were also out, and they lost control of the mobile apps (couldn't make the apps show that trains didn't run, the in-app scheduler showed all was A-OK), and that sounds quite dissimilar from the core train operation service, making me think it's more of an infrastructure than a specific system's problem. On the other hand, once the app could be controlled again (assuming this singular underlying cause), you'd think they could then also start putting train service back in place and that didn't happen for hours still.

I can't really make the pieces fit together for any theory, so then presumably something multi-faceted (one thing tripping one or two other things so it escalated from restarting one component to not being able to restart the trains anymore the whole day).