Perhaps it's a bit far-fetched, but at Frankfurt Süd at the end of the s-bahn platforms there are two square stones with "TBC" on them... could this mean to be continued, which will be an easter egg for a tunnel extention???
So its confirmed that DTG wont make an extension for Frankfurt (anyone said that it got confirmed on railsimforum), but that doesnt mean TSG wont do it. The asset is from Niddertalbahn, a TSG route, so this either is a very nice hint or entirely missleading.
that could very well be a sign for an extension which would be made whenever the 420 releases for FTF, as a theory.
