I am a little confused by your "My hope is to get a version of the D&RG 4-4-0s modified to represent the C&Ls first two 4-4-0s which were nearly identical excepting a few hardware differences." At least according to the EWRH the Harrington, engine #2 was a 4-4-2 although I have found some errors in the book. I, as yet, do not have access to the Baldwin Locomotive Works data so I can't be of much help there.
The 4-4-2 in question is actually a 4-4-0. You prompted me to remember that I forgot put upload the spec sheets to PacificNG. Here they are.
Both locomotives are built to roughly the same design with the second being a bit more modern having been built a good number of years later. For all intents and purposes their closest neighbor would be the D&RG 4-4-0s like D&RG #95 which is available on the DLS. The detail differences would be a "short" smokebox and a Radley-Hunter stack, everything else would be close enough.
As for Northern Railway equipment, the entire picture is definitely murky for the early days. By time of the consolidation in 1888 the Northern Railway really only existed on paper as another part of the CP/SP empire. According to the corporate history for the line in the back of "A Century Of Southern Pacific Steam Locomotives" the original 1871 operation never owned rolling stock of it's own and used CP/SP equipment. This stays true until the consolidation and later years when the SP orders rolling stock lettered for it, though they wore the standard SP "sunset route" herald on the side. I've started talking to Curtis about some proper CP/SP rolling stock as he and I need the same cars.
I suspect the locomotives listed in EWRH are the narrow gauge locomotives that fell under the Northern Railway numbering, these were originally from the SJ&SN and the Oregonian lines. I do not have a copy of EWRH myself as I tend to have the source materials that author references on hand.
What numbers does he list for those locomotives? I am curious about that.
I noticed that neat little repaint of the 2-4-0 before, it's a shame the CP/SP never used them (or that wheel arrangement). Though the Visalia Railroad did have a 2-4-0 which was later owned by the SP apparently (per some rosters) it would have been a bit more modern than the Bowker design having been built in 1886. Sadly the CP/SP locomotives will take more time to get accomplished as most of their equipment was heavily modified in Sacramento during this time. Few locomotives are "identical" in design save a great deal of classes which simply are not available in Trainz. Much like my modeling exploits IRL (V&T circa 1900) the core CP/SP freight locomotive is missing from the scene: the 4-6-0. I might be able to work something close enough from a model of Sierra #3 if one is available.
Now all of this would be for naught if I didn't include enough of the line to operate on... I suppose it was a good idea to leave the terrain in place and construct the trackage from Williams to Maxwell.
The Northern Railway crossing is an important part of the C&L experience as all trains were required to stop at the crossing before proceeding, and the Colusa/Colusa Jct side operated multiple round trips to pick up travelers at the junction. I couldn't leave it unmodeled!