You can't link el-trains or subway with normal trains.
You can not, at least not with the buildings that come with the game. (You can do almost anything if you are willing to download buildings.) If you take a close look, you can see that the subway trains and el-trains look the same, and the normal trains look different. This is modeled after real life, normal trains came first, then el-trains and subway both developed as mass transit options, using similar trains/tracks. If you take a look at some real world mass transit systems, you will see switching between subway and elevated rail. The example I know of off the top of my head is San Fransisco, Ca's BART system.
I believe the best alternative is to place a station of each type next to each other. If I recall correctly, the sims can walk between the two to transfer, and be on their way. This is not as efficient, because the sims don't like to change transportation modes anymore than necessary, and if your transit system is stretched, it may not work well at all. This option also mirrors the real world, see for example BART/Amtrak's Richmond, Ca Station.
If you click on that last link, you'll see that Richmond station is at ground level, meaning of course that the real world DOES have mass transit tracks/trains on the ground. There's nothing preventing us from doing it the real world of course. However, ground level mass transit is simply not common, (Bart heading away from Richmond raises to an el-train shortly) and regardless, its not in Sim City 4.