Lionel 6-28069 smoke unit.

How close is the heat sensor (thermistor) to the smoke resistor?  A picture of the disassembled smoke unit showing the distance between these 2 components will help us see if they are too close and preventing the circuit from making the resistor hot enough.

Here's a link to more discussion on how the distance between these two can affect smoke output.

Thermistor Question

EDIT: Caution is also advised not to make them too far apart, else the resistor can overheat causing damage.  Picture would help.

@SteveH posted:

How close is the heat sensor (thermistor) to the smoke resistor?  A picture of the disassembled smoke unit showing the distance between these 2 components will help us see if they are too close and preventing the circuit from making the resistor hot enough.

Here's a link to more discussion on how the distance between these two can affect smoke output.

Thermistor Question

EDIT: Caution is also advised not to make them too far apart, else the resistor can overheat causing damage.  Picture would help.

These do not have thermistors.  Too early vintage.

@MartyE Thanks for the info.

One thing also to consider getting replaced if not already done would be the plastic gears. These engines suffered from having plastic gears that would chew up or break entirely if I'm not mistaken. I had Alex M do the work on fixing my gears as well as the smoke and chuff(4 per Rev) to the brass gears that make it pull out stumps with that Pittman motor.