One thing to consider is your insurance if things do go wrong. If you read the small print under Missed Departure in your policy it might surprise you, in some cases it will horrify you!
Margaret may not trust public transport but most policies have fewer exclusions related to public transport than private car - which suggests the insurers do trust it. Taxis usually get lumped in with private cars.
One common exclusion is that whilst you may be insured if you're delayed because your car is
involved in a crash you're not if you're car is stuck behind a crash on a motorway or dual carriageway (and therefore can't turn round). And where car delays are covered it's often hard to get the documents to support the claim.
With public transport most insurers will accept any failure to meet a published schedule (if it was a sensible choice - don't pick a train that is advertised to get there 5 minutes before checkin closes!) and all you need is a letter from the operator confirming it.