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+## 11. Stop Transfers (transfers.txt)
+
+*This file is ***optional*** in a GTFS feed.*
+
+To define how passengers can transfer between routes at specific stops
+feed providers can include `transfers.txt`. This does not mean
+passengers cannot transfer elsewhere, but it does indicate if a transfer
+is not possible between certain stops, or a minimum time required if
+transfer is possible.
+
+| Field  | Required? | Description |
+| :----- | :-------- | :---------- |
+| `from_stop_id` | Required | The ID of stop as it appears in `stops.txt` where the connection begins. If this references a station, then this rule applies to all stops within the station. |
+| `to_stop_id` | Required | The ID of the stop as it appears in `stops.txt` where the connection between trips ends. If this references a station, then this rule applies to all stops within the station. |
+| `transfer_type` | Required | `0` or blank means the recommended transfer point, `1` means the secondary vehicle will wait for the first, `2` means a minimum amount of time is required, `3` means transfer is not possible. |
+| `min_transfer_time` | Optional | If the `transfer_type` value is `2` then this value must be specified. It indicates the number of seconds required to transfer between the given stops. |
+
+It is also possible that records in this file are specified for
+ticketing reasons. For instance, some train stations are set up so that
+passengers can transfer between routes without needing to validate their
+ticket again or buy a transfer. Other stations that are shared between
+those same routes might not have this open transfer area, thereby
+requiring you to exit one route fully before buying another ticket to
+access the second.
+
+### Sample Data
+
+The following table shows some sample transfer rules from TriMet in
+Portland's GTFS feed (<https://openmobilitydata.org/p/trimet>).
+
+| `from_stop_id` | `to_stop_id` | `transfer_type` | `min_transfer_time` |
+| :------------- | :----------- | :-------------- | :------------------ |
+| 7807           | 5020         | 0               |                     |
+| 7807           | 7634         | 0               |                     |
+| 7807           | 7640         | 0               |                     |
+
+These rules indicate that if you are transferring from a route that
+visits stop `7807` to any route that visits the other stops (`5020`,
+`7634` or `7640`), then this is the ideal place to do it.
+
+In other words, if there are other locations along the first route where
+you could transfer to the second route, then those stops should not be
+used. These rules say this is the best place to transfer.
+
+Consider the transfer rule in the following table, taken from the New
+York City Subway GTFS feed (<https://openmobilitydata.org/p/mta/79>).
+
+| `from_stop_id` | `to_stop_id` | `transfer_type` | `min_transfer_time` |
+| :------------- | :----------- | :-------------- | :------------------ |
+| 121            | 121          | 2               | 180                 |
+
+In this data, the MTA specifies how long it takes to transfer to
+different platforms within the same station. The stop with ID 121 refers
+to the 86th St station (as specified in `stops.txt`). It has a
+`location_type` of `1` and two stops within it (`121N` and
+`121S`). The above transfer rule says that if you need to transfer
+from `121N` to `121S` (or vice-versa) then a minimum time of 3
+minutes (180 seconds) must be allocated.
+
+If you were to calculate the time taken to transfer using the
+coordinates of each of these platforms, it would only take a few seconds
+as they are physically close to each other. In reality though, you must
+exit one platform then walk around and enter the other platform (often
+having to use stairs).
+