So, where do we find the Flow of Goods (FoG)? Click on the barrels icon in the upper left (or hotkey G on PC):

Here is what you see upon opening FoG. I added some red boxes and numbers to help with the breakdown of what you see:
The FoG shows us all the commodities (both raw materials and finished goods) and mail & passengers. Mail & passenger have different info displays, so we will address them separately. The default view is always on Grain (or its equivalent depending on DLC/scenario) Using the above picture let’s breakdown what the FoG gives us:

1. Name of commodity.

2. Total production and demand for that good – this is global. This is assumed to be in carloads per month or week, as extrapolated from the city demand info panel (not depicted). Finally, shows the split between how much is transported by rail versus wagon. In this picture we can see there is room to grow in transporting grain by train!

NOTE: All numbers depicting carloads shipped can be expressed in terms of Months or Weeks. This is map/scenario dependent. Hovering the cursor over the status icon and referencing the tool tip in the lower left will help you decipher what timescale is used.

3. Listing of raw materials and finished goods.

4. Demand info status icon. Demander is red, a supplier is green. A city may be a demander or supplier. A city is only supplier of finished goods – such as Clothing. Rural resources are suppliers only. The commodity icon is on top of the status icon. The crates and number indicate number of carloads per month/week consumed by the city. The smiley face gives a graphical happy or sad status based on the percentage of demand satisfied, which is the final number depicted.

5. Supply info status icon. A city is the only supplier of finished goods – such as Clothing. Rural resource are suppliers only. The commodity icon is on top of this status icon. The gears and number indicate number of carloads per month/week produced by the supplier. The crates number is the carloads in storage.

There are two types of arrows that emanate from the supplier to the demander:
6. A large arrow indicates flow by train.
7. A small arrow indicates flow by wagon.

8. This arrow indicates the direction of a supplier that is outside the current view. You can ‘navigate’ to that supplier by keeping this arrow near center-top of the map as you scroll. This will give you a sense of how far off the supplier is in order to decide to use it or not.

Warehouse inventory is displayed in a gray icon showing carloads on-hand:

Hover your cursor over a city and the supplier icon changes. The green supplier icon changes to display the number of carloads shipped to your city. The bar below shows the percentage of those shipments that is sent by train:
The picture above depicts 16 carloads being shipped weekly on average over a month/week period. Here all Grain is being shipped train. That’s what we like to see!

If you click on the city icon, a yellow circle appears on the city. This indicates locking the view to that city. This is useful as the shipping status indicated above is only visible when hovering over the city. By locking on the city, you can move the map without losing the supplier focus:

Hovering the cursor over city, we see the Passenger relationship (Mail works the same). Again, arrow size differentiates between wagon (small) and train (large) with the bar under the status icon indicating the percentage of train use. As the tip box in the lower left corner states, the number next to the gears in the status icon are Passengers per month (or weeks) traveling from the selected city. You can lock the city as discussed earlier in the guide.

Satisfying supply and demand is a core function of the game. The Flow of Goods is a useful tool to help solve this puzzle by:

1. identifying how well demand is satisfied and what commodity your city needs to grow;

2. allows you to plan better when selecting factories (or dealing with AI factories) for your cities by helping to identify resources near the city versus across the map, and identify your customers.

Now go forth and conquer!