Ok... The 2035 is a fairly "heavy-duty" loco with smoke and magne-traction. When you factor in a heavy whistle tender and 5 lighted cars, that's a lot of drag! So 10 volts isn't excessive to get the train moving, especially if your loco might need cleaning or lubrication. Is the loco spry and responsive by itself, without the train?
As you increase the thumbwheel on the DCS remote, the voltage on the screen will probably begin to read higher than what's actually going to the track. Don't be too concerned about the digital readout, it's a proportional approximation. MTH's bricks put out 22 volts so at 18V the Lionel Powerhouse PH-1 is down a little from that. It should still be adequate, but going through the TIU, not all of that voltage will be available at the rails. If in doubt check the voltage on the rails with a meter.
Some other thoughts: Look over the tender and your passenger consist. Spin each wheel with your finger. If necessary put one! (1) drop of oil on each bearing surface where the axle goes into the wheel, and where the axle meets the truck. You can do this on the pickup rollers too, but be sure to wipe off any excess.
If the track is brand-new, running your train will burnish its surface and friction may be reduced so it might run faster or more responsive. Make sure the track is plugged together tightly for good electrical conduction. Also, that wheels and rollers don't "snag" on the gaps between sections. This can cause the train to stall at very slow speeds.
If you really want more speed and power to spare, you would need a 22-volt MTH brick, a 180-watt Powerhouse, or a top-tier transformer like a ZW, ZW-L, or Z4000. But if your passenger car windows are glowing white hot, you might be applying too many volts and your loco is probably in need of maintenance. Hope this helps!