I am lucky enough to live near the Katy Trail. I live 18.4 miles from the east end of it. I rode there and back yesterday. It's about the same distance from my house to the trail as it would be from the Alton, IL train station to the end of the trail, and the route would be approximately the same.
The trail ends, or begins, depending on your perpective, in Machens, MO. There ain't much in Machens, and to tell you the truth, the place looks a bit inhospitable. See for yourself:

As you can see, the sign says "No Katy Access." Well, you can get on the Katy from there. I've even parked my truck right near that sign and ridden to St. Charles. I don't understand why that sign is there. The trail end is about 1/4 mile from Machens, in a desolate spot.
However, there is a gate at the end of the trail and there is a working bathroom, and there is a kiosk with a bench and some maps and a welcome sign.
I tried to go a little ways towards St. Charles yesterday but the trail was soft and my tires sunk in and I had to turn around and head home. It is 12.6 miles from Machens to St. Charles.
The ride from Alton to Machens is very doable but a bit dicey. The high traffic areas like the Clark Bridge have shoulders but there is a good bit of debris. You want good tires.
Once you get into Missouri and off the highways, you ride on highway 94 which is a two-lane country road with no shoulder. The traffic is light. I had a stiff westerly headwind going out. Coming back with the tailwind was very nice.
Much of my touring is on the Katy. I try to do the whole length once/year, and I do many day trips and some overnighters.
That is all.