There was a swedish chap at phase 1 last month (and indeed phase 2 in a few weeks time) who flew to Dusseldorf (presumably from Stockholm) and then got trains from there to Maastricht. Its a fiddly train journey, i think he said he took 4 trains. Its not actually that far, only about 80 miles i think, but multiple changes are needed. Remember to keep in mind that it will probably be much easier to fly back the following day. Airport to train station is about 20 minutes and then you obviously have a few hours of trains to catch so flying back on the evening of the tests would be a mission. Besides, you'll get the chance to go for a beer or 8 in Maastricht (theres some nice bars!)
Me, for phase 1 i flew to Amsterdam from Birmingham, then trains to Maastricht. Train is €25 each way, and takes about 2.5 hours (3 trains). Thats what im doing for phase 2 also.Flying to Maastricht direct is unlikely unless you have your own aircraft!