Employers pay you for the time you are actually at work (barring sick/holiday/special leave etc for the purposes of this discussion). How you get to/from work, mode of transport used and time taken is neither here or there as far as the employer is concerned (unless your job role specifically involves travel).
Quite. There's often some give-and-take, depending on the job - if you normally start work on time (and no employer has a right to expect more) and make every reasonable effort to be on time when there is disruption, employers will generally be as understanding as possible. Obviously some jobs allow more leeway than others. If you show up late every day and your only comment is 'Sorry, the train is never on time' or 'Sorry, bad traffic today', then your employer is likely to take a rather dim view of affairs. Even if the train
is always late, or the traffic
is bad during the rush hour, it's your responsibility to plan accordingly.
My only caveat to that would be, if you start a new job where you're not wholly familiar with which trains are late or which roads are busy, a bit more leeway might be reasonable. Most people will probably make sure they're early on the first day, but on the second day perhaps not so much. After the first week or so, though, the expectation should certainly be for the employee to be on time.
From the employee's point of view, of course, the expectation should be that public transport can stick to the published timetable! PPM statistics are all well and good, but a train that is 4 minutes and 59 seconds late isn't going to be considered 'good enough' by the passengers. Although it would be interesting to see PPM statistics developed on the same basis for bus and coach operators.... In my experience, buses would be doing well to make it into double-digit percentages in some places.