measure voltage drop

Since you're running conventionally, that would be AC voltage.  Same with Legacy and DCS.  The wall wart that comes with some Lion Chief sets is DC.

To be on the safe side, make sure that the meter is set to a range greater than or equal to your supply voltage, eg. 20VAC at first for each measurement location.  If the measured drop is very small and you need greater resolution in certain locations, then try a lower range there and switch back to the higher range before moving to the next measurement location.

You will need a train or some other load in place where it is slow. Track voltage measurements are almost meaningless, without a load. Some lighted passenger cars will present a decent load.

If I move the MTH 400e to various places and leave it in neutral does that constitute a load? If not I can pull the passenger cars out of their box again

How big is your layout? and how many transformer connections to the track do you have? Once you get beyond the average 30"x "50 starter set loop you need to think about adding extra track feeders.

You say it slows down on the opposite side of the track with the lockon, this is pretty typical. With out even measuring the voltage you could simply add a lockon at the point of slowdown and see if it improves.

Technically yes there would be some load with just the Loco in neutral, but not nearly as much as when the motor is running nor as much as incandescent lights in many passenger cars.

The greater the load, the more of a measurable voltage drop you will be able to read across a particular resistance.

An easier way to load the track consistently is to use a large resistor to simply add a known load to a specific point on the tracks.  I use some clipleads and one or more of these honkin' 75W resistors to give me a known load.  This makes it much easier to determine where the issues are.  You can just measure the drop through specific track joints, you may have one or two poor connections.  Finally, another effective method is to place the fixed load at the far end and then check track joints for any high temperature, any poor connections will be hotter than good connections.

That looks like a plan. When I google 75w resistors, I did find something that looks like this. The ads talk about various ohm values. What am I looking for?

Well, mine are 8 ohms, that's because they were left over from many years ago, load resistors to test audio amplifiers.

And two 8s can make 4 or 16.  Is an 8 ohm load similar to the load of a typical O Loco?

8 ohms would pull about 2 amps from 18 volts by the time a few connection drops figure in.  That would be a bit more than a can motored locomotive unless smoke was in use.  My Vision Line Big Boy with four smoke units pulls around 4 amps with a light load and all the smoke features enabled.

Thanks John.  As usual good info.

@gunrunnerjohn posted:

Well, mine are 8 ohms, that's because they were left over from many years ago, load resistors to test audio amplifiers.

Yes, I have those too, stored right next to my vacuum tubes.

@shorling posted:

Yes, I have those too, stored right next to my vacuum tubes.

Mine are stored next to my germanium rectifiers.

@Oman posted:

Mine are stored next to my germanium rectifiers.

Well mine are stored next to the galena crystal with cat's whisker

...next to the Voltaic piles...

Now that we've all had our belly laugh, do you suppose we could get back to assisting the OP?

Certainly. Voltage drop happens at the opposite side of all layouts. It is to be expected, due to the less-than optimal conductivity of steel versus other metals. Thousands of model railroaders have installed a second lockon® to mitigate this problem.  Additional reasons for this voltage drop (actually the cause is cumulative resistances) are loose track connections and length of the track diagram.