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Step 4: keep things under your control = maps and roster sheets
I know very well there are players that can keep memory of all trains, routes, etc., but I am not like this: when I have more than 10 trains I start to loose control of my network For this reason I always keep an updated map of the whole system and a roster sheet.
My maps show all railroad stations and major trolley/bus stops. I use different symbols for stations with passenger (a round) and freight service (a square); from this game I start to use a new symbol: a star; this means a generic plant without commercial service like engine house, yard, passing loop, junction, and so on. Of course, the map shows also all lines with different colors and size to specific different transportation systems (standard and narrow gauge railroads, electric and steam/diesel operation, trolley lines, etc.). Here is the map of the northern portion of the Bay Minette game in 1910; the brown lines are the Powder River RR, the red the city streetcar in Powder.

- 1910-PRRR-map.png (41.8 KiB) Viewed 5267 times
In the meantime I keep also an updated roster sheet where I list in rows ALL trains, vehicles, ships and planes. For each vehicle I indicate:
Category: train, trolley, etc.
Subcategory: steam, diesel, electric.
Reporting mark: the company name's initials (I like to simulate more than one transport company, all run by me under the same player) and number of the vehicle.
Type: the name/classification of the actual type of the vehicle (i.e. for a steam loco 4-6-0 or 4-6-2, etc.).
Cargo: the cargo or cargoes carried by the vehicle.
Route code: i set a code (with numbers and/or letters) for every route; vehicles with the same route code must have shared orders. In fact I divide the route code in two columns: the first indicates the category (express or local passenger, mail, freight, etc.), the second the route itself. So, for example, a local and an express passenger train running exactly on the same line have a different route category but the same route "number". Codes must be indicated in the map, too.
Route: the first, last and eventually main intermediate stations of the route.
Notes: useful to indicate something that it's important to not forget.
Usually I add route codes when the network is becoming really complex; as a matter of fact I don't introduced them in Bay Minette game, yet, as you can see from my roster sheet:

- 1910-roster.png (23.48 KiB) Viewed 5264 times
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You know very well my philosophy "keep as few tracks as possible". Here is an example how I apply it. Look at Powder South Junction:
no flyovers, no double tracks, no parallel switches, I kept the junction and the surrounding tracks quite simple. By the way, they are used by seven trains:
2x Stockton Mines – PWPCo 1 coal
1x Stockton – Point Clear passenger
1x Clear Bridge Farmers Co-op – Powder Brewery grain
1x Daphne Mines – PWPCo 2 coal
1x Powder – Daphne passenger
1x Powder Elevator Spur – Powder Brewery grain
and, believe me, I never had real jams!