This needs to be seen in the context that there is two unions in the train sector. The first is the EVG which is large, corrupted (high ranking union officers getting nice jobs at the company after leaving the union) and generally reluctant to fight. The train drivers founded the GDL to actually have a fighting union and since then most of the train drivers joined them instead of EVG.

The GDL aims to also get to negotiate contracts for other functions within the train sector, but the allegdegly social democrats passed a law to kill smaller unions, by forcing them to accept the deal made by the larger union if two unions are present in the same site. This law was challenged and upheld by the constitutional court under the condition that the positions of the smaller union are adequately represented by the larger union too. This is highly questionable in the context of the train sector in Germany.

So this is a critique by a larger partly compromised union organization reluctant to fight, against a smaller and willing to fight union.

Also the DB, the largest and state owned train company in Germany didnt even start formal negotiations until after the last contracts ran out.