There is support for this idea across MPs of multiple parties, from Conservatives, Liberals, Greens and Labour/Left. It appears to be a comparatively cheap measure to reduce CO2 footprint (I am not an expert here, though, so this might be incorrect, but one hears this argument a lot) and at the same time an easy measure to improve mobility for the poor. (Thousands of people went to jail in Germany every year because they repeatedly travelled without a ticket and could not or prefered not to pay the panalty fee.) So the zeitgeist is very much in favour of it, and ecological topics (in the wider sense) are becoming more and more important for people's decission what party to vote for. So what initially were niche topics of the Greens are becoming increasingly popular and more so, when they coincide with goals of other parties, such as social policy ideas from Labour/Left and the trade unions or the administrative simplifications ideas of the Liberals. Note also that Germany is already heavily subsidising its local puplic transport, with the effect that everyone pays taxes for tickets that only a minority uses.[1]

[1] At a moment's notice, I could only find old estimates from 2008 that amount to 25 billion Euros per year (more than 300 Euros per inhabitant per year) for local and regional transport (not including long distance transport) as of which only 9 billion Euros (35%) were paid directly from tickets or similar. This means 16 billion (65%) were already payed by the public (aprox. 200 Euro per inhabitant per year) -- see: https://www.zukunft-mobilitaet.net/wp-content/uploads/2013/0... (in German).