These sorts of concessions are granted to many hundreds of thousands of railway staff nationally, including some astonishingly large entitlements for 'protected' staff who were employed pre-privatisation, all of which have been set in stone for decades. They're also valid on and issued by the most heavily subsidised operators such as ScotRail, Northern, Cal Sleeper and Merseyrail depending on exactly which measure you choose. Lots of office staff who actually have very little to do with the running of the railways, or staff at some tourist attractions which happen to be railways are also given various travel benefits. There are also retired and dependent entitlements. I don't see any complaints from you about any of this though given the benefits are plainly going to be worth many times over £15 million per year? And what of the fact that offering such benefits may actually be especially shrewd because it partly makes up for pay being uncompetitive against Network Rail or consultants in many TfL roles?30,000 plus ones for TfL staff £10p.w, £15m.
Your view is entirely blinkered, focusing on some small opportunity costs and making up that the Bakerloo line is being used as a hostage.
Even collectively these things are all trivial vs the real cause of the financial problems, which were the three main factors I point out above.So what is your definition of a trivial sum of money?
Lots of people have now explained this lots of different ways.