Breaking it down...

Dear LD,

Let's take a short walk thru some "old skool" mech diagnostics.
(They're oldies, but goodies, and can save your sanity and mechs...)

Firstly, it sounds like you've already done a basic
- place both units on a common piece of track, about a yard apart,
- slowly throttle up the analog throttle "speed"

and observe the comparative speed/behaviour/response

If the speeds are greatly different, then yes, this can pose issues when running coupled-together.

If there is enough weight to keep the wheel/rail mechanical relationship
(IE the wheels are _not_ sliding on one or both units)

then the difference in speed/torque is being "taken up" by the mech and motor. This can cause mechanical stresses in the driveline, and in some cases can manifest as electrical issues. (motor being restricted in it's movement = drawing excess current, motor being "forced to spin faster than expected" = generates excessive BEMF, etc)

If however, there is _not_ enough weight to keep both unit's wheel/rail relationship together,
(IE slower unit is sliding it's wheels from being dragged, or faster unit is sliding it's wheels from being held-back),
either way, the wheels are sliding, and are likely wearing away excessively at the rails and the wheel surface. (Check for a recent thread on a Athearn CF7 which was running poorly, and the owner was about to start tearing apart and rewiring the trucks, only to discover that the _actual_ fault condition was the NS plating having been worn off the wheel treads, likely due to similar wheel-slippage issues).

Having confirmed and ascertained which unit is running slower, let's ask some basic questions:

Does the slower unit:
- have a DCC decoder installed?
(a decoder equipped unit on analog will have a higher starting-voltage than an equivalent analog-wired unit,
and may have a different track-voltage--> track-speed response on analog)
- have any form of constant lighting circuit installed?
(even a basic diode-dropping constant-lighting circuit can affect apparently-identical locos)
- have any kind of mechanical issue?
(mechanical binding, such as scenery wrapped around an axle, wiring rubbing against the driveline,
Excess oil getting into the motor/commutator+brushes area, excess thick/sticky/old grease in the trucks,
all can cause mechanical restriction or "excess load" on the mech)

Assuming and hoping desperately that you're comfy enough with taking the shell off the mech, and confirming the above, we _should_ end up at a position where:

- "excess" electronics have been eliminated as a cause of excess volt/amp draw
- motor is not affected by undue outside issues (oil ingress, etc), and is essentially "operating at spec"
- all parts of the mechanical drivetrain have been checked for binding, alignment, and basic operation
(should be smooth without noticeable friction, no "binding points" at any stage in the rotation, and little/no "excessive slop" backlash, esp around any worms)

If we've gotten to this point,
- the 2 locos _still_ are't running "near enough" to the same,
- and you're _committed_ to running them "as a usually-coupled consist",

then they may simply need some "speed matching".

"but you can't do speed matching without DCC?" 

Not true. Try this:
(Assuming a loco that os _literally_ wired for _pure_analog_ running,
IE Left rail/wheel -->  LH motor terminal> Motor> RH motor terminal> Right wheel/rail)

- get 2x 1N4002 sillicon diodes
(should be < $0.10 ea via RadioShack or similar)
- solder them "back to back"
(IE so that the silver bands ends of each diode body are _not_ connected together)
- drop one (does not matter which) of the motor wires on the _faster_ unit, and insert the resulting two-diode assembly in-circuit

Bingo, you've just UP-ed the "starting voltage" of the faster unit by 0.6V,
and effectively _lowered_ it's top speed (and flattened it's speed curve from 0-12V on analog).

Retest the paired units by doing the "on same track at 1 yard apart" test, and see if this has made things better.
You can continue adding additional diode-pairs until the units are "reasonably closely" matched...

EG I recent had to match a Steam Era Models "Black Beetle" and Fleishmann "Magic Train" mech in a pair of steam-dummy locos. The Black Beetle ended up with 4-pair of diodes, to drop it's response to match the Magic Train...

Hope this helps...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

Prof Klyzlr: Thanks for the

Prof Klyzlr:

Thanks for the information. I'll take your advise and determine why the supposedly two identical cars do not operate identically. Neither is suppose to have a decoder in it but who knows for sure without looking. If all else is right then I'll match the speeds with series diodes to drop the voltage to the motor of the faster car. As these are expensive models as HO models go, I wanted to make sure I was not harming them running them tandem without matching their speeds.

LDBennett

Further notes

First off, I would suggest that you look into the reason the two do not run together.  "Identical" models should run together as well as anything on the market, and mass production should not result in that much variation.  As Prof suggested, look for additional electronics or mechanical issues in the slower model.  I would even suggest going as far as a full tear-down, cleaning, and reassembly with proper lubrication.

If, after all that, the models still do not run close to the same speed, the diode circuit may be the best option.  If you find that you need two pairs of diodes, you could use a bridge rectifier and install a constant lighting circuit - might as well, really.

Another option, if you think you might ever want to run these models on a DCC layout and if you have a friend or local shop with DCC programming abilities, would be to install inexpensive decoders and speed match them that way.  That advantages would be that you could then run them on DCC any time you like, and you can more carefully match the speed curves, adjusting at least the start, middle, and peak voltages, and sometimes (if you're so inclined) many, many points along the curve.

Both cars are relatively new,

Both cars are relatively new, with the slower one the newest. So my thought was maybe the newer one was not broken in. I ran it continuously for several hours and then stuck both on the same track but not coupled. The "slow" one had become the fast one (??).

What I have seen in all my seven trolleys is that cold out of storage they have to warm up to run at full nominal speeds. I think this is the case here as the warm car was faster than the cold one.

The "slow" one had no decoder. When I did finally couple them up I found that my CREST throttle (a pulse width modulator of a regulated commercial DC power supply) and the 3.5 amp power supply would ever so slightly surge at startup and surge going through switches. When going through switches I think one of them looses good contact with the track causing the effective voltage to surge from the Crest throttle and make a surge in speed.

The bottom line is running them in tandem probably is not my best choice. To do it on my tight layout turns (it is a trolley layout) I had to add long shank couplers to keep the car bodies from rubbing in the corners. Add to that the surgeyness running and I think it not worth it.

Thank you everyone for your inputs.

LDBennett

Dear LD, Understand about the

Just had a thought ...

(Yeah, I know - that could be dangerous!)

Based on the experience others on the web log have recently had, perhaps, if you clean the gear trains on your locos and then use the Nano Oil (an advertiser here), you may solve the problem with speed-when-cold versus speed-when-warm. Who knows? Maybe both will perform better and be matched.

Another challenge!

There is another fly-in-the-ointment for my particular situation. 

After doing some testing and running the two cars together, I parked them on their designated siding. I have a siding for each trolley to store the trolleys (and these interurban cars) on the layout. Each siding is powered through a toggle switch so once on there, I turn off the siding power. Well, I never planned on having two interurban cars. Its siding is long enough physically but part of one of the cars is on the main block and that car is not turned off with the toggle switch.

I had not considered taking the motor out but had considered just getting an unpowered car. I may just put the newer powered car back in the box and save it for later in case something happens to the other one. Theses interurban cars run so smoothly on my layout compared to some of the trolleys that I tend to run them more often and may break or wear out one.

Thanks all for the comments.

LDBennett