The problem with extending the Heartland Flyer to either Kansas City n is there are no spare Superliners around to add a second train set to this train. It runs today normally with just one set of 3 to 4 Superliner cars.
The existing set round trip lasts 13 hours over a total distance of 412 rail miles. There's 11 hours remaining within 24 hours idle in Oklahoma City. The highway distance between Oklahoma City and Newton is 186.5 miles, a round trip would be at least 373 rail miles - most likely more as the tracks take a longer route than I-35. It's only 39 total miles shorter. In its favor is that the Heartland Flyer lays over in Fort Worth for 5 hours, so there's plenty of padding in the schedule.
Assuming it takes just as long to reach Newton as it does to reach Fort Worth, which is 4 hours, the Flyer would be arriving in Newton around 1:30 am. To be back in Oklahoma City at the time it is now scheduled to depart, it would have to leave Newton around 4:30 am. So it could have a 3 hour layover in Newton.
As a reminder, the Heartland Flyer would be in Newton between 1:30 am to 4:30 am, in the wee late night hours. The Southwest Chief is scheduled today to be in Newton at 2:45 am westbound and 2:59 am eastbound. Which is basically in the middle of that 3 hour window for the Heartland Flyer. Which brings up the question, is there enough tracks and platforms in Newton to handle all three trains at once?
Kansas City is another 200 rail miles from Newton, 400 rail miles per round trip; so extending the Heartland Flyer to Kansas City will require an additional train set. The same answer applies for extending the Heartland Flyer to San Antonio, which is 283 rail miles away from Fort Worth, 566 rail miles per round trip. If Amtrak was to extend the Flyer to Newton or San Antonio, you'll be expected to change trains in Newton just like you do or will do in Fort Worth today.
Oklahoma and Texas revenues have taken a hit with the fall of oil and natural gas prices. Neither state is in a position to double down on subsidizing inter city trains. So what may be physically possible isn't fiscally possible right now.