The doodlebug was a "heavyweight" (1920s-era) coach converted by some railroad lines for use as a kind of self-propelled bus on tracks. Seen here is an Aristocraft (that's a "toy train" manufacturer) Milwaukee Road combine (combination baggage and coach car) doodlebug that I use to test the track. It was the first one on the outdoor rails this spring.
Above: Leaving the CRD through the portal (click for larger view).
Below: The doodlebug emerging on the outside of the CRD (click for larger view).
Here is a close-up view of it parked in the model town of Cicely, Alaska:
It made a successful run about two weeks ago in advance of the sending of the first large train consist down the tracks yesterday. Currently the bug is parked in Cicely.