You're misunderstanding my point. Transport subsidy funded from current income is a good idea. Spending your entire capital infrastructure budget on making it free for a few years is not. The 'lingering benefits' don't linger long if you then have to hike fares massively because you've got a big loan to repay as a result with no new assets you can exploit to service it.

It's for this reason that saying 'we could spend the money on making buses free for a N years' is meaningless. It would be more sensible to say 'we should spend the £Nbn we spend on maintaining the motorway network on free buses' because those types of spending are equivalent.