Stan
Not a great idea. Used to be more popular when motors drew more current, rheostats were cheap in comparison to rectifiers, and smooth operation just wasn't common.
Variable resistor control works very poorly today because most - but not all - locomotives have high efficiency can motors. The older open frame motors would typically draw 0.3-0.5 amps in operation on a layout, with Athearns going as high as 0.7 amps. A variable resistor of 40-50 ohms was needed to be able to control these. The rheostat needed to be able to dissipate as much as 10-15 watts running the train at slow speed. Wire wound potentiometers (variable resistors) were typical due to power requirements and smooth resistance changes.
A newer can motor drawing 0.15 amps cannot be run at slow speed by a 50 ohm rheostat. In this case, the rheostat can only drop 7.5 volts at most, which leaves 4.5 volts as your starting voltage. Most modern locomotives will jump into action with 4.5 volts applied. If you use a 100 ohm variable resistor to control the can motor, then you have to turn the variable resistor to at least half scale to start the older locomotive moving.
For the above reasons, direct transistor control of output voltage is preferred, and is available in better commercial power packs like the MRC Tech series.
Next issue with your independent sections is when the train crosses the section boundary from one section to another. At that point, the 2 rheostats are in parallel, which reduces the total circuit resistance to half the total of the 2. The locomotive will suddenly speed up until it is wholly contained in one section again, where it will slow back down with another jerk. Metal wheels on cars which bridge the rail gaps on both rails simultaneously will have the same effect. If the direction switches happen to be opposite when the locomotive bridges the section - instant short circuit.
Last problem with section control is that buying/finding appropriate variable resistors is niether trivial nor cheap anymore. They just aren't made. Before heavy acceptance of transistor throttles, the preferred rheostats were wire wound, non-linear tapered resistance, with at least 270 degrees of rotation - all in the interest of better control.
These short-comings of section control led to the development of block control. In block control, each train has its own power pack that follows it wherever the train goes on the layout. Each block, or electrically isolated section of track, has an electrical switch which selects which power pack will control that particular train. This avoids any problems unless you accidentally get 2 trains inside the same block at the same time. When that happens, both trains go in the same direction and at whatever speed the track voltage gets them. You have simply lost independent control of the trains. Setting up block boundaries correctly is key to operational flexibility while avoiding 2 trains in the same block.
In block control, the power packs don't have to be the same, and can be as feature-laden or not, as one desires. They only have to have enough power to run one train - 12 watts is sufficient unless you are double-heading Athearns or have a long trains of lighted passenger cars.
yours in DC block wiring
my thoughts, your
