I did expect to fly the whole SID and the reason I didn't wonder (loudly) where the hell it is taking me is that I was expecting radar vectors sooner or later. I've had a radar departure at certain large East European airports, with vectors given within a minute of takeoff. Anyway, a SID is not a radar departure.
Not checking that the SID connects to my filed route was a big mistake.
I would have started to query the route when approaching the end of the SID, when a near-180 turn would have been needed to get onto the filed route. Not being familiar with that airport (or Germany really, except enroute) I assumed that they assigned that silly SID due to the heavy departing traffic, for separation of IFR and the much more heavy VFR traffic.
Re comms failure, I should know the rules for German airspace but don't..... In the UK, it is the last assigned heading for 7 minutes and then revert to filed route, no? In the USA, it is the clearance, then the EFC (which doesn't exist in Europe) then filed route. In this case, I would have flown the SID to the end, climbing 5000ft, then turned towards TRA which was the filed route. But I have a battery powered radio anyway, for this kind of eventuality. What should one have done in this case? The SID is the clearance so you have to fly it whole. Do you then fly the last heading of that SID for 7 more mins? What if there is terrain?
A good lesson in several ways.