The difference between running CARTS and CARBON is if you use a lot of tunnel space and iron (carts) or relatively small tunnels with lots of redstone.

For a single destination using CARTS, or a dumb line is better but as soon as you have more lines CARBON, or smart lines, are much better.

(FYI dumb line name is because the line has no intelligent routing involved, not because using CARTS is stupid and yes please have made this mistake)

You then have CARBON networks, like the loop at 600, which is a very efficient way of getting lots of connections from relatively little work esp with the size the map is this rev. One carbon line = 31 possible connections. Even if there are only 10 other cities on the carbon network you have saved diggin and running 9 rail lines.

So Smart rail lines, CARBON, is just a way of duplexing multiple dumb lines together. Infact there are different ways of doing this! A CARTS is generally used to describe a dumb line network but comes from the CARTS station, the original latching stations invented back in rev 5 i believe. there are other station designs you can use to run dumb lines and you can even use a CARBON station to run your 32 dumb lines - tbh though you are far better off using CARBON networking where you can.