About 5-10% of a flight, according to this:

https://www.google.com/amp/s/amp.dw.com/en/trains-vs-planes-...

Massively https://www.withouthotair.com/c20/page_128.shtml

Yes. This graph from "Sustainable Energy – without the hot air" is a good summary: http://www.withouthotair.com/c20/page_128.shtml

Yes, about ten times as low.

See (for one): https://www.seat61.com/CO2flights.htm

I guess it depend if the train is running on electricity or diesel, but in France I had read an article in a french newspaper stating that the carbon footprint of train was 10% of the one from planes.

This article seems to have the same numbers: https://www.seat61.com/CO2flights.htm