This is common in the UK. Many people work for/are involved in the public transport service, so it gets treated as plural and takes a plural verb. 'It's' would be equally valid and also sounds natural to me, there you would be treating the service as one separate body and therefore it would take a singular verb. This second way is I believe preferred in American English. It's just a slightly different way of thinking about things.
What doesn't sound natural to me is when things like sports teams get referred to using a singular verb, which like I said above is the convention in American English (unless the name of the team ends in a 's', then they seem to switch to use a plural verb) i.e. 'Manchester United is playing well'. In the UK, they almost always take a plural verb in that situation... Manchester United are playing well.