Add images

gtfs-realtime/spec/en/reference.md

@@ -617,7 +617,6 @@ The stop times of each replacement trip are created from those of the affected t
 Frequency of update of the data is expected to be a few times per day (1-12/day).
 Ingesting time on the data consumer side is expected to be about an hour (<2 hour).
 
-
 **Fields**
 
 | _**Field Name**_ | _**Type**_ | _**Required**_ | _**Cardinality**_ | _**Description**_ |
@@ -635,6 +634,12 @@ A `Modification` message replaces a span of n stop times (`num_stops_replaced`)
 
 The sequence of `new_stops` may be of arbitrary length. For example, 3 stops could be replaced by 2, 4, or 0 stops as the situation may require. 
 
+<img src="images/trip_modification.png" width=200px style="display: block; margin-left: auto; margin-right: auto;">
+An example showing the effect of a modification on a particular trip. This modification may also be applied to several other trips.
+
+<img src="images/propagated_delay.png" width=200px style="display: block; margin-left: auto; margin-right: auto;">
+Propagated detour delays affect all stops following the end of a modification. If a trip has multiple modifications, the delays are accumulated.
+
 
 **Fields**
 
@@ -651,6 +656,8 @@ Each `ReplacementStop` message acts as a template for generating a new stop time
 The `arrival_time` is calculated from a reference stop in the original trip, plus the offset in `travel_time_to_stop`. The `departure_time` always equals the `arrival_time`.
 The optional fields of [`stop_times.txt`](https://github.com/google/transit/blob/master/gtfs/spec/en/reference.md#stop_timestxt) in the GTFS-Static specification are all set to their default values.
 
+<img src="images/first_stop_reference.png" width=200px style="display: block; margin-left: auto; margin-right: auto;">
+If a modification affects the first stop of the trip, that stop also serves as the reference stop of the modification.
 
 **Fields**