Our son went to college/grad school in St Louis and we experimented with all sorts of public transport in getting him between out house (NW suburban Chicago) and his school (in the CWE part of St Louis). We found that while flying was the most expensive alternative it was also by far the most convenient and on-time. My son took Amtrak a total of three times and he was never on time. Two of the delays were marginal (30 - 60 minutes). But one involved a freight train on their track and was several hours in delay. This was in the 2006-2014 time frame. I have read that there have been improvements recently (in the last 2 years) along with a promised increase in train speed (up to 110 mph).

We are (at most) 20 - 25 minutes from ORD and the tme for him to get from school to the airport was at most 30 minutes. So if you include a two hour check in window at STL, he was something like 4 to 4,5 hours away. The scheduled train ride was (and still is) 5.5 hours. Assuming normal check in and connection times to and from the relevant train stations, the total trip time would be something like 8 hours, at an absolute minimum

If you are going from city center to city center, the train could be very workable for you. Otherwise, depending on the distance travelled between source/destination and the relevant train station, you will likely find that flying will be more convenient but more expensive.