Ten years and 3.5 million passengers later, Amtrak has something to celebrate this week. Thursday marks the tenth anniversary of the Downeaster-passenger train service between Boston and Portland that has exceeded even the most enthusiastic expectations. A celebration that began with a pizza party on Monday for Amtrak employees will culminate Thursday with an anniversary ceremony in Portland. Meanwhile, a short distance up the road in Brunswick, construction workers on Tuesday lowered into place several sections of prefabricated concrete to form the train ramp at Brunswick Station. It’s the next stop on the return of passenger rail service to Maine, projected to expand to Brunswick in late 2012.