Max grade for MTH subway set?

The El on my railroad has two loops, and each as two 6% grades. I find 6% is the maximum limit for the MTH Rail King IRT LoV and BMT Q train sets. The LoV cars are heavy, so I had to scuff the track to increase traction.

Extensive running on grades with six cars makes my MTH motor cars excessively, and in my view unacceptably hot. 

Something to think about. The MTH unpowered subway cars are essentially dummy locomotives, with the same truck blocks as the PCC and Brill trolleys. If the trucks had the same friction characteristics as freight cars, no problem climbing steep grades. But subway trains are really one car hauling locomotives--minus the motors, spur/worm gears and electronics.

That's why when I ran 10-car trains, I had two power cars in the mix.

---PCJ

Thanks all for the responses.

Bobby, any over heating problems running your subway continuously? 

Tommy, extensive running on what % grade?

FWIW, This will be for frequent display running  point to point with multiple station stops using 4 cars total.

The grades on my El are set by the MTH Real Trax pier set 40-1033 which is advertised as having a 4.7% grade. However, I shimmed some of the piers so the grades are between 5 & 6%.

Since scoring the grade tracks I have been running a 6-car LoV set, and I do notice the train slows down after a few minutes of running. I assume the slowdown is a result of the motor heating or the transformer power dropping. Power and control of the El trains is by Z500 Transformers, one on each El loop. Each loop has 64 feet of Real Trax, so the Z500 Transformers are taxed to their maximum. Z1000 Transformers will replace the Z500s.

In my opinion, a 6-car LoV train needs to have a second motor car and I am on the hunt for a used car or just the power truck. My set has Loco-Sound, not PS2. The only complaint I have about the LoV set is that the window glazing has fallen out on several of the cars.

Spark simulators are eye-catching attractions for onlookers. The simulators light when a wheel connects an isolated segment of outside rail.

Isn't the formula for the grades 1/4" per foot?

Grades require 2 power units for 6 -8 car trains on O42 - O54 tracks or to will stress the 1 PWR unit. Plan on changing a lot of tires. Clunky trucks are just too heavy for the outdated design.