This DLC is actually a double pack; It includes the EMD DD35 and the GP35. Since these locomotives share the same features, the following critique will directly apply to both.
First Impressions:
Both locomotives look very detailed, with high resolution textures inside and out. Some players with less than beefy computer rigs may find they get some lag when panning around the locomotive. Roof-top fans are animated. Windows and doors open and close. Dynamic weather effects for windows are included. Windshield wipers work. The locomotives have engine kill switches in the cab. The locomotives are both equipped with advanced braking features (awesome). The cabs are equipped with a manual emergency brake handle on the Conductor's/Fireman's side that works (very nice). And, oh my word....The locomotives have air hoses that connect when the locomotives are coupled. And....the included rolling stock has air hoses that connect. And the air hoses on the cars connect to the air hoses on the locomotives....kind of a big deal.

Sound Effects:
The prime movers sound like EMD 567's to me. Dynamic brake cooling fan FX is present but a little on the quiet side. Air-brake bail-off, bell, and sander (at least inside the cab) are audible and sound good. The air-horn sounds good and the sound file allows short blasts for signaling purposes. Control surfaces in the cab make all the requisite thunking, clicking, and sliding sounds. Coupling and uncoupling sounds are solid.
Lighting FX:
Tons of goodies here, as we have come to expect from DTM. Headlights, roof-top strobe light, and numberboard lights are included. Class lights work and can be changed between white, green and red. Cab lights and instrument lights work. Step-lights work. All of these features are independently operated and fully functional.
Performance:
All I can say is that the locomotives seem like they have proper tractive effort and horsepower curves. The only way to know for sure is to dive into the game files and examine their figures. However, in the scenarios I have for the locomotives, they seem like they can lug tonnage and get up to track speed. Your mileage may vary.
Extras:
This add-on comes with four scenarios. More are available in the workshop.
As a bonus, it comes with rolling stock: a 40ft Boxcar, Flatcar TOFC with four loads, 100t Hopper, Grain Hopper and CA-4 Caboose, all of which look (to me) period-appropriate for the 1960's-1970s era. This addition really adds value to an already great package.
Nitpicks:
Locomotive lacks snow FX (snow on walk-ways and blowing snow from the undercarriage). It would have been awesome to see that during a Sherman Hill blizzard. Locomotive exhaust looks fuzzy and pixelated and does not vary with throttle setting. And while the locomotives have animated brake cylinder travel and brake rigging ( a very nice feature), the animations only work on the lead locomotive and only when coupled to cars, and only when using the train-line brake application. The animations do not work at all when the locomotive is running light, or when the independent brake is applied (wether m.u.ed with other locomotives or running light), or on the trailing locomotives of a m.u.'ed set. The outside volume of the dynamic brake is too low, needs just a little adjustment. Can't hear the hiss of the sander outside the cab. Windshield wipers do not remove precipitation from the GP35's Engineer and Fireman-side front windows, but they do work for the center window. Dynamic brakes do not show loading on the ammeter (HUD ammeter) (a problem shared with the DTM SD50). Engine trucks could perhaps have shown more detail. Included caboose lacks interior views/interior lighting and exterior lighting, i.e. end of train lamp or class lamps. And finally, locomotives do not include realistic start-up procedures.
Pros:
-Two locomotives in one pack with included rolling stock
-The usual high quality we have come to expect from DTM
-Iconic, unique, and rare loco (DD35)
-This locomotive, due to it's vintage (early 1960's) would compliment the GP30, DD40, Big Blow, SD45, and F.E.F very well for making 1960's era Sherman Hill scenarios.
Cons:
-No advanced start-up procedure
-No snow FX
-The rest are nit-picky and outlined above
Conclusions:
Overall, this release from DTM is an excellent value. When you take into account that you get two brand new locomotives with rolling stock, and the detail and operating features these locomotives have, they are hands-down a better quality product than most of the usual stock locos that Dovetail Games makes in-house and bundles with its North American routes with perhaps the exception of the General Electric U30B that came with the Feather River Canyon Route DLC. I look forward to future DTM locomotive releases with great anticipation. I would definitely recommend this DLC to my friends. Oh... I'll give it a 4.75/5 stars.....