Strikes

December is the month of strikes in Hungary.

The state owned railways (MAV) is on strike now for more than 3days, some of the trains are going, some aren't, and there are really noone who can tell the actual timetable.
So, try to avoid trains in these days, catch a bus instead, at Nepliget or Stadionok station. But you have to be ready that buses may overcrawded.
When the strike will end? Hopefully within days...

Ferihegy airport , security workers have a strike, they try to slow the procedure of check in, long queues..more waiting. You have to be in the airport a couple of hours before your departure time.

BKV, the Budapest Transit Authority is possibly on strike within days, so the mass transport system in the capital city will be a mess...
Taxis are a good alternative, but they can easily stuck in the traffic jam , due that many resident will take car instead of the bus/tram.
You can walk or rent a bike! It is not that cold yet...