Hello everyone, I was concerned that my Desert Route won't use Carrizo Gorge and Mexico due to sharp curves. Is there's another path where I could lay track that has some sort of "transcontinental railroad" feel, from Plaster City to El Cajon directly?

Err...no?
I think there's a UP cutoff between some point on the Sunset Route and El Centro, but that might be totally unrelated.

Look at a highway map from Plaster City to El Cajon... The highway doesn't go straight, and a highway can climb a lot faster than a railroad. Look at a terrain map - Plaster City is near Sea Level and El Cajon is at about 900 feet, but the mountains go to over 5000 feet in about 50 straight-line miles between the two. Spreckels and Co. used the best possible route for a railroad without major (as in miles-long) tunneling. That has been the major limitation of the Desert Line; modern long cars and three-axle truck locomotives just don't fit on the curves and in the tunnels, and you can't get around the problem.

Hmm, try making the route as you go along, if you have to make the route have a 3.3% grade, DO IT! But are you asking for a real route that exists or a fictional route?

The route will be designed as semi-real and semi-fictional. I've also completed track laying in the Rice to Ripley branch, a fictional extension to Palo Verde, and a part of the trolley line between Santee and somewhere in the Orange Line.