Well, it's more likely you'd work in reverse - identify, collaborate with, or be a company that is planning a large scale facility that can make use of rail freight such as a new mine, an electric steel mini-mill, or (very likely) a transfer station for C&D/MSW - by necessity these would be short lines (or perhaps even captive lines, as in minehead to power plant rail-loop), but they would be new railroads (ladies & gentlement, I present Long Island's newest railroad, the
Suffolk & Southern - I love the exasperated tone the STB seems to have in that document concerning people filing notices of exemption for all sorts of odd reasons, and their bafflement at whether Suffolk & Southern plans to run trains over that 10,000 feet of new track or not, and even if that track exists
: Actually, I guess in a week we'll find out if Southern & Suffolk will still exist. Still, that filing gives you an idea of the kinds of reviews and paperwork you'd need to start a new short line).
Now, starting a new Class I in North America... I dunno - DM&E is having an heck of a time getting financing for their Powder River expansion, and that projects proven to have interested customers (maybe not enough to justify the expansion). ROW acquistion would be dang expensive, and persuading enough customers to use your service would be a nightmare.
So...I can see it be possible to start new Class III shortlines to serve a particular customer or group of customers, which would then interchange with other railroads, but a brand new Class I...hard to conceive of...