Slow running 040 saddle tank

Yes! You are running an analogue loco on DCC. That is a good way of destroying the motor, because DCC is pseudo AC!

The loco is 'seeing' very lumpy 7.5 volts at max speed, instead of the 12 it is designed for.

I know the 'book' says that you can run an analogue loco with a DCC controller in code zero, but if you read the forums, they will all say the same thing - DON'T!

Just because you can do something, doesn't make it a good idea i.e running a DC loco on a DCC track. You have been very lucky so far not to damage the DC loco motors. You may have already done some low level damage to the motors without even realising it. It's not worth the risk, either fit or have fitted some cheap DCC decoders.

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There is a lot of electrical theory [Zero Bit Stretching] involved with manipulating DCC voltage waveforms to drive a DC loco. It is not straight forward. See these links for further reading.

Damage to DC Motor

DC Loco on DCC Track.

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PS - Also, when operating your controller in 'DC Zero Mode' any DCC locos running at the same time may become erratic in control because of the way that the DCC signal waveform has been modified to get the DC loco to run at all. Operating in Zero Mode is not a good idea on very many levels.

As the others have said, it's not a good idea to run a DC loco on DCC. The motor can burn out in seconds, especially if it is stationary. Far better to fit a decoder. The loco will still work on DC, but the DC controller will have to be turned up a bit higher. DCC fitted locos start moving at around 3 volts.

Where do you get the 7.5v figure from 2e0?

I've got a few DCC locos poliss and they move off at the same setting as the DC ones. Perhaps it depends on the controller. 

After a time I removed the decoders and plugged in the blanking plates. Made no difference at all. Did to somebody though, I gave them to someone on the modelrailwayforum. 

Perhaps you found no difference, WTD, because you were not feeding those locos with DCC current allowing loco 0 DC control.

Incidentally, do engine drivers ever ride the type of tank loco under discussion "side-saddle?"

Poliss is talking about DCC running on DC Graskie. Have another read of his post. 

Poliss wrote  - Where do you get the 7.5v figure from 2e0?

Half the nominal 15 volts of the pseudo AC voltage - assuming the motor is 'seeing' pulsed half-wave output from the controller. I've never bothered measuring it with a meter or sillyscope to see!

As I understand the output to the track, the loco will be getting 15 volts pseudo DC from the bit stretching, when the controller gives it full throttle. Which is why backontrack's loco going slow seems odd to me.

Whichever, the best thing to do is still to fit a decoder. :-)

A DC loco on DCC sees and acts upon the modified 'biased DC' voltage as applied by the throttle, but it feels the standing DCC voltage all the time, which is why they get hot and buzz, i.e. the full unbiased +/_ wave DCC voltage passes through the motor in both directions alternately hence no movement at zero throttle.

Poliss is talking about DCC running on DC Graskie. Have another read of his post. 

Yes, I knew that, WTD, but you obviously referred to DCC locos running at the same speed on your DC layout, with a DCC decoder or blanking plate fitted, as opposed to correct thread talk about running DC locos on a DCC layout.

Please reread your own post ;-)