Short numbers 
- I don't prefer huge numbers. Anytime I end up with number longer than 3 digits, I feel like I failed in a game design. The reason is that player easily get lost when numbers like $ 103157.62 appears and usually it means that I was not able to balance the game so I used every "scale" from $ 1 to $ 1 000 000 000 to achieve scalability for different game parts / epochs.
- On the other hand, scalability itself is strongly needed in any kind of game, where you either level up (and enemies became stronger) or go through many epochs like in this game, from steam age to modern age. You need to differentiate prices in different game stages so player can buy almost anything at the beginning of the game, but to buy thinks in later game phase, he needs to earn huge pile of money from his transport company. Otherwise, player is not motivated to make money at all and there would be very little challenge. For this game, I've found the solution by introducing what I call Technology tokens.
Technology tokens 
- Every economy strategy game is about earning some money and investing them to earn even more. In this game, money will be the very first "token" you have, but once you start to deliver some cargo, you will also earn other Tokens. For example when you deliver logs from forest to saw mill, you will earn small amount of Wood Tokens. You can spend them to buy new small steam engine, invent better steam engine or build wooden bridge, build some wooden upgrade to one of your train stations etc. In each history epochs, there will be one more token type introduced and it will be more difficult to earn them, but also more interesting things to buy.
- I like control over the game progress, and this includes research of any new technology. When the game gives you electric engines just because it is already the year when these engines have been invented in the real history, it is nice surprise for the player and it adds to the realism, but the history of locomotives inventions was written by locomotives manufacturers. Therefore I decided, that in this game, you will be also one of them. I prefer the approach, where you need to invest to the research and invent new engines by your self by spending some of these Technology Tokens. For this specific case, you need to deliver fuel to electric plan to earn Electricity Tokens and unlock some better electric locomotives.
- Tokens are not supposed to just unlock new tech, you will need to decide where to spend them. You can for example build better bridges with Steel tokens, or unlock better wagons. With Coal tokens, you can for example refuel steam engines, or unlock new steam engine type.
- It also kind of auto balance rule in the game. When you don't have enough Tokens to buy current trains, you would not unlock even more expensive trains. On the other hand, when you don't need to invest to your train fleet, you can spend some Tokens to unlock new tech. There will be always more possibilities, you can either keep your fleet small but modern and effective, or use cheaper but huge fleet of trains, or upgrade your infrastructure (stations, bridges, tunnels, ..)
- In each of 8 epochs, one new Technology Token will be introduced. Every token is defined by its epoch (for example in diesel epoch, you will have Diesel Tokens which unlock diesel engines etc).
- Thanks to this approach, every time you have lets say 400 of one type of tokens (so the prices will be already 3-digits), there will likely be the new token introduced, way more valuable, where each one will make the difference.
Upgrades 
- I love upgrades:) It gives you possibilities to customize your strategy, it also gives you the control over game progression. So I made a game mechanics from it.
- Upgrades for Stations – When your station is used as a passenger train station in the middle of a town, you can extend it by Hotel and increase income from every waiting passenger. You can buy Parking space to extend range radius from which passengers will came to your station. When you have cargo station, you can speed up loading and unloading with Crane upgrade, increase stock capacity with Warehouse, etc.
- Upgrades for Industries - When your business is based on some specific industry, you can spend some tokens to upgrade it. It is up to you. For example you can build Steam Saw extension to the existing Saw mill to speed up production but you would need to deliver not just logs, but also small amount of coal to keep this upgrade working. On the other hand, you can build Charcoal hut extension to existing Forest so it will produce small amount of coal. Then you don't need to connect Saw mill with Coal Mine, but only add coal wagon to your logs train. Possibilities are endless.
- Upgrades for Depot - You need depots to build or manage you trains. Depots are able to shuffle wagons and buy limited types of new engines and wagons. Although I like technology trees, I decided that one different approach fits better to this game: When you would like to have better engines and wagons, you need to build new building next to your depot. It could be for example Diesel engines assembly line, so you will be able to build new diesel locomotive there. Since this upgrade will be quite expensive, you would need to drive new engine to it's line, or build this extension to more depots. It is up to you and your rail network situation.
- Upgrades to trains - Although this is still something I have to properly design, the idea behind upgrading train is to allow player further customize his trains by buying special wagons. For example, you can buy dining car which will act like Station upgrade Restaurant, but each time you deliver passenger to any station. Of course, such wagon will not be cheap and will make the train longer, slower,... so you have to think twice. Most of these special wagons will be available only as a reward by completing some of "quests".
Randomized "quests" 
- When player knows all rules and the game goes strictly along one direction, he can easily get bored. I like the idea of random subsidy (connect two towns and get twice more money for a limited time period), but I haven't use them in these games, because it was only for limited time which usually runs out before I finish the track to accomplish it :) Therefore I decided to support lua scripting language and prepare some randomized events with bigger variability and totally different impacts.
- For example, lets say there will be a quest, where some amount of unique cargo appears in one of your stations, some unique wagons in one of your depots and you have to deliver the cargo by those wagons to some specific station / location. You will get not just money out of it, but for example one "prototype" of not-yet-invented locomotive, special wagon, or get some unique reward. Possibilities for quests are huge, and I would like to support some of them by a small back story.
- This is also great way how to integrate tutorials into the game, since player is not forced to go through these, if he is already skilled player, but he is motivated by the unique reward to finish it.
Freely editable terrain 
- For such sandbox game, it is quite vital to allow player modify terrain freely. It comes with the price, but you can modify terrain to allow you some better station or track placement. You can also create unforgettable landscape scenery or bring sea to the town by digging long channel.
- I like customization of the terrain, so you will be able to plant trees, bushes and some buildings. When you are in the editor, you can also place industry, prepare some scripted events and etc.
Realistic view 
- I like board games a lot, but since I was render programmer on few AAA projects, I also like the immersion of realistic look. To deliver both, I worked hard to combine realistic look and schematic map, similar to board games. At the end, I realized, that it needs to have two separated modes and switching between them. One for realistic look, second for construction mode. And it works perfectly. Player can easily thing about the situation, but at the same time, he can let himself enjoy what he accomplish, ride in one of his trains.
1st Person 
- The main genre is a sandbox strategy, but I was always thinking of how it would be to jump into one of your train and taste the game as from a simulation genre in 1st person perspective. This combination feels very unique. As a strategy player, you are kind of level designer, build junctions, change the landscape, connect hills by bridges... But when you switch into driver mode, you kind of play your level, you have to navigate across the entire railroad network to deliver the cargo. It is completely different perspective and it will create new challenges, new possibilities and a new world of fun :)
