diff --git a/docs/sandbox/TransmodelApi.md b/docs/sandbox/TransmodelApi.md
index f44a21d5313..dcf96d1e723 100644
--- a/docs/sandbox/TransmodelApi.md
+++ b/docs/sandbox/TransmodelApi.md
@@ -49,6 +49,8 @@
   [#4161](https://github.com/opentripplanner/OpenTripPlanner/pull/4161)
 - Add possibility to filter dated service journeys by replacementFor
   [#4198](https://github.com/opentripplanner/OpenTripPlanner/pull/4198)
+- Add support for groupOfLines in top level query
+  [#4232](https://github.com/opentripplanner/OpenTripPlanner/pull/4232)
 
 ## Documentation
 
diff --git a/src/ext/java/org/opentripplanner/ext/transmodelapi/TransmodelGraphQLSchema.java b/src/ext/java/org/opentripplanner/ext/transmodelapi/TransmodelGraphQLSchema.java
index a0eb0751ddc..1eb813fd5bd 100644
--- a/src/ext/java/org/opentripplanner/ext/transmodelapi/TransmodelGraphQLSchema.java
+++ b/src/ext/java/org/opentripplanner/ext/transmodelapi/TransmodelGraphQLSchema.java
@@ -757,7 +757,7 @@ private GraphQLSchema create() {
 
             if (stops.isEmpty()) {
               return new DefaultConnection<>(
-                Collections.emptyList(),
+                emptyList(),
                 new DefaultPageInfo(null, null, false, false)
               );
             }
@@ -1183,6 +1183,35 @@ private GraphQLSchema create() {
           })
           .build()
       )
+      .field(
+        GraphQLFieldDefinition
+          .newFieldDefinition()
+          .name("groupOfLines")
+          .description("Get a single group of lines based on its id")
+          .type(groupOfLinesType)
+          .argument(
+            GraphQLArgument
+              .newArgument()
+              .name("id")
+              .type(new GraphQLNonNull(Scalars.GraphQLString))
+              .build()
+          )
+          .dataFetcher(environment ->
+            GqlUtil
+              .getTransitService(environment)
+              .getGroupOfRoutesForId(TransitIdMapper.mapIDToDomain(environment.getArgument("id")))
+          )
+          .build()
+      )
+      .field(
+        GraphQLFieldDefinition
+          .newFieldDefinition()
+          .name("groupsOfLines")
+          .description("Get all groups of lines")
+          .type(new GraphQLNonNull(new GraphQLList(new GraphQLNonNull(groupOfLinesType))))
+          .dataFetcher(environment -> GqlUtil.getTransitService(environment).getGroupsOfRoutes())
+          .build()
+      )
       .field(
         GraphQLFieldDefinition
           .newFieldDefinition()
diff --git a/src/ext/java/org/opentripplanner/ext/transmodelapi/model/network/GroupOfLinesType.java b/src/ext/java/org/opentripplanner/ext/transmodelapi/model/network/GroupOfLinesType.java
index 8154a2ca74d..c316168b2e9 100644
--- a/src/ext/java/org/opentripplanner/ext/transmodelapi/model/network/GroupOfLinesType.java
+++ b/src/ext/java/org/opentripplanner/ext/transmodelapi/model/network/GroupOfLinesType.java
@@ -2,9 +2,11 @@
 
 import graphql.Scalars;
 import graphql.schema.GraphQLFieldDefinition;
+import graphql.schema.GraphQLList;
 import graphql.schema.GraphQLNonNull;
 import graphql.schema.GraphQLObjectType;
 import org.opentripplanner.ext.transmodelapi.mapping.TransitIdMapper;
+import org.opentripplanner.ext.transmodelapi.support.GqlUtil;
 import org.opentripplanner.transit.model.network.GroupOfRoutes;
 
 public class GroupOfLinesType {
@@ -62,6 +64,17 @@ public static GraphQLObjectType create() {
           .dataFetcher(env -> ((GroupOfRoutes) env.getSource()).getDescription())
           .build()
       )
+      .field(
+        GraphQLFieldDefinition
+          .newFieldDefinition()
+          .name("lines")
+          .description("All lines part of this group of lines")
+          .type(new GraphQLNonNull(new GraphQLList(new GraphQLNonNull(LineType.REF))))
+          .dataFetcher(env ->
+            GqlUtil.getTransitService(env).getRoutesForGroupOfRoutes(env.getSource())
+          )
+          .build()
+      )
       .build();
   }
 }
diff --git a/src/main/java/org/opentripplanner/routing/graph/GraphIndex.java b/src/main/java/org/opentripplanner/routing/graph/GraphIndex.java
index 7953aaee8bb..c853471b8fc 100644
--- a/src/main/java/org/opentripplanner/routing/graph/GraphIndex.java
+++ b/src/main/java/org/opentripplanner/routing/graph/GraphIndex.java
@@ -27,6 +27,7 @@
 import org.opentripplanner.model.calendar.ServiceDate;
 import org.opentripplanner.routing.vertextype.TransitStopVertex;
 import org.opentripplanner.transit.model.framework.FeedScopedId;
+import org.opentripplanner.transit.model.network.GroupOfRoutes;
 import org.opentripplanner.transit.model.network.Route;
 import org.opentripplanner.transit.model.organization.Agency;
 import org.opentripplanner.transit.model.organization.Operator;
@@ -58,6 +59,8 @@ public class GraphIndex {
   private final Map<ServiceDate, TIntSet> serviceCodesRunningForDate = new HashMap<>();
   private final Map<FeedScopedId, TripOnServiceDate> tripOnServiceDateById = new HashMap<>();
   private final Map<TripIdAndServiceDate, TripOnServiceDate> tripOnServiceDateForTripAndDay = new HashMap<>();
+  private final Multimap<GroupOfRoutes, Route> routesForGroupOfRoutes = ArrayListMultimap.create();
+  private final Map<FeedScopedId, GroupOfRoutes> groupOfRoutesForId = new HashMap<>();
   private FlexIndex flexIndex = null;
 
   public GraphIndex(Graph graph) {
@@ -101,6 +104,12 @@ public GraphIndex(Graph graph) {
     }
     for (Route route : patternsForRoute.asMap().keySet()) {
       routeForId.put(route.getId(), route);
+      for (GroupOfRoutes groupOfRoutes : route.getGroupsOfRoutes()) {
+        routesForGroupOfRoutes.put(groupOfRoutes, route);
+      }
+    }
+    for (GroupOfRoutes groupOfRoutes : routesForGroupOfRoutes.keySet()) {
+      groupOfRoutesForId.put(groupOfRoutes.getId(), groupOfRoutes);
     }
     for (MultiModalStation multiModalStation : graph.multiModalStationById.values()) {
       for (Station childStation : multiModalStation.getChildStations()) {
@@ -244,6 +253,14 @@ public Map<Station, MultiModalStation> getMultiModalStationForStations() {
     return multiModalStationForStations;
   }
 
+  public Multimap<GroupOfRoutes, Route> getRoutesForGroupOfRoutes() {
+    return routesForGroupOfRoutes;
+  }
+
+  public Map<FeedScopedId, GroupOfRoutes> getGroupOfRoutesForId() {
+    return groupOfRoutesForId;
+  }
+
   public HashGridSpatialIndex<TransitStopVertex> getStopSpatialIndex() {
     return stopSpatialIndex;
   }
diff --git a/src/main/java/org/opentripplanner/transit/service/DefaultTransitService.java b/src/main/java/org/opentripplanner/transit/service/DefaultTransitService.java
index ef089d18071..e591460559c 100644
--- a/src/main/java/org/opentripplanner/transit/service/DefaultTransitService.java
+++ b/src/main/java/org/opentripplanner/transit/service/DefaultTransitService.java
@@ -34,6 +34,7 @@
 import org.opentripplanner.transit.model.basic.WgsCoordinate;
 import org.opentripplanner.transit.model.framework.FeedScopedId;
 import org.opentripplanner.transit.model.framework.TransitEntity;
+import org.opentripplanner.transit.model.network.GroupOfRoutes;
 import org.opentripplanner.transit.model.network.Route;
 import org.opentripplanner.transit.model.network.TransitMode;
 import org.opentripplanner.transit.model.organization.Agency;
@@ -393,6 +394,21 @@ public Collection<TripPattern> getPatternsForStop(
     );
   }
 
+  @Override
+  public Collection<GroupOfRoutes> getGroupsOfRoutes() {
+    return graphIndex.getRoutesForGroupOfRoutes().keySet();
+  }
+
+  @Override
+  public Collection<Route> getRoutesForGroupOfRoutes(GroupOfRoutes groupOfRoutes) {
+    return graphIndex.getRoutesForGroupOfRoutes().get(groupOfRoutes);
+  }
+
+  @Override
+  public GroupOfRoutes getGroupOfRoutesForId(FeedScopedId id) {
+    return graphIndex.getGroupOfRoutesForId().get(id);
+  }
+
   /**
    * Get the most up-to-date timetable for the given TripPattern, as of right now. There should
    * probably be a less awkward way to do this that just gets the latest entry from the resolver
diff --git a/src/main/java/org/opentripplanner/transit/service/TransitService.java b/src/main/java/org/opentripplanner/transit/service/TransitService.java
index 75c850a6de8..cc94d857881 100644
--- a/src/main/java/org/opentripplanner/transit/service/TransitService.java
+++ b/src/main/java/org/opentripplanner/transit/service/TransitService.java
@@ -27,6 +27,7 @@
 import org.opentripplanner.transit.model.basic.WgsCoordinate;
 import org.opentripplanner.transit.model.framework.FeedScopedId;
 import org.opentripplanner.transit.model.framework.TransitEntity;
+import org.opentripplanner.transit.model.network.GroupOfRoutes;
 import org.opentripplanner.transit.model.network.Route;
 import org.opentripplanner.transit.model.network.TransitMode;
 import org.opentripplanner.transit.model.organization.Agency;
@@ -138,6 +139,12 @@ List<TripTimeOnDate> stopTimesForPatternAtStop(
 
   Collection<TripPattern> getPatternsForStop(StopLocation stop, boolean includeRealtimeUpdates);
 
+  Collection<GroupOfRoutes> getGroupsOfRoutes();
+
+  Collection<Route> getRoutesForGroupOfRoutes(GroupOfRoutes groupOfRoutes);
+
+  GroupOfRoutes getGroupOfRoutesForId(FeedScopedId id);
+
   Timetable getTimetableForTripPattern(TripPattern tripPattern, ServiceDate serviceDate);
 
   TripOnServiceDate getTripOnServiceDateForTripAndDay(FeedScopedId tripId, ServiceDate serviceDate);