Code 172 represents very heavy rail.
code 148 would be more appropriate for typical mainlines and code 125 for sidings. Some guys even use some code 100 for industrial and light traffic areas.
Brass is not as good as either Nickel silver or steel. While brass is a very good conductor, the oxide is not conductive. So once the rail heads oxidize a little, contact gets bad. Nickel silver is not so good a conductor, but the oxide is conductive. so if the rail oxidizes a little, you still get good contact.
I don't know much about steel, A number of the big O scale clubs use it. I do know that it is less slippery for traction than Nickel silver. Conductivitity is probably someplace between brass and nickel silver.
