"Background: I love NARS and the Add-On set, but I don't like their lack of focus. It feels weird to me to have PRR duplexes, CP Jubilees, and SP Cab Forwards all running on one railroad. Additionally, there are some serious gaps in their steam line up. My favorite era is right around 1920, and I usually keep my game within a few decades of that year for its entirety. (Either cheating the date or using a long day length.) This works fine as is, but the problem is that old engines don't get updated anymore. In NARS 2.03, steam engines would evolve over time, so a 2-8-0 built in 1914 has far stronger stats than the one built in 1868. In NARS 2.51, you have old time wood burners, then some updated coal burners, and then it goes straight into modern road engines like mikados and berkshires, etc.
"The problem with this is that when I'm starting my game in 1920, I have to choose to either way overpower my trains with giant (for the time) engines like 2-8-2s and 4-6-2s, or buy 1800s era engines that look out of place hauling current cars and have awful stats.
"My solution is to create a new pack that goes into much more detail, offering various flavors of different wheel arrangements from different eras. For example, the first 4-4-0s looked very different from the express engines of the late 1800s, which looked very different from the later superheated versions. This is true across almost all wheel arrangements.
"My method of execution would be to take my favorite railroad, the Southern Pacific, choose representative classes from each wheel arrangement and era, and then create models for each class. Many of them would look the same, many would have very similar stats, but you'd get a huge amount of variety while maintaining a very uniform family look. My sources would be steamlocomotive.com for stats and espee.railfan.net for photos.
"I'd also like to make freight and passengers cars to fit the SP theme, including variations such as diners, sleepers, RPOs, etc. I have some ideas for this, which I'll go into if and when I make them.
"I'd also like to include their narrow gauge trains, and maybe some classic stations in SP colors as well."
Well, I've got too much time on my hands, so here's my first version! It's mostly a proof-of-concept and a proof-of-my-ability-to-actually-finish-and-release-things, so I only have one locomotive so far: the SP A-3 4-4-2. The A-3 is my favorite, and I also happen to be building an N scale model of it, so I have a lot of notes on its exact construction, as well as a model to reference for drawing sprites. I've implemented it in seven liveries, created a custom repaint cargo, and implemented dynamic running costs. So I'm pretty happy with this and ready to keep going! Next will come a caboose, then probably some passenger cars, then a light freight engine like a mogul, probably an M9.
The license right now is restrictive, since it's barely out the door, but eventually I plan to release it freely and I'm also open to collaboration. I have 68 standard gauge engines and a number of narrow gauge ones planned, so more hands would not be unwelcome.
Screenshot:

Enjoy!