1. A communication device in a train network system that includes a plurality of hubs (11, 21, 31, 41, 51, 61) installed in train vehicles, each of the plurality of hubs (11, 21, 31, 41, 51, 61) belonging to both a trunk network in which a signal transmission range is not limited and a branch network in which a signal transmission range is limited, and adding, to a signal to be transmitted to a hub belonging to the same branch network as it does, network type information indicating whether the signal is a trunk network signal or a branch network signal, the communication device operating as one of the hubs (11, 21, 31, 41, 51, 61),
the communication device transmitting branch network information including at least identification information on a branch network to which it belongs at a predetermined timing, and based on branch network information received from an adjacent communication device, determining whether or not to add the network type information to a signal to be transmitted to the adjacent communication device.
2. The communication device according to claim 1, wherein the branch network information includes information necessary for setting to join the branch network.
3. The communication device according to claim 2, wherein when the communication device in a state of not having joined a branch network receives the branch network information from an adjacent communication device, based on the received branch network information, the communication device determines a branch network to join and performs setting of the branch network.

4. The communication device according to claim 1, wherein the trunk network and the branch network are realized by VLANs, and the network type information is a VLAN tag.
5. A train network system comprising a plurality of hubs (11, 21, 31, 41, 51, 61) installed in train vehicles, each of the plurality of hubs (11, 21, 31, 41, 51, 61) belonging to both a trunk network in which a signal transmission range is not limited and a branch network in which a signal transmission range is limited, and adding, to a signal to be transmitted to a hub belonging to the same branch network as it does, network type information indicating whether the signal is a trunk network signal or a branch network signal, wherein
each of the hubs (11, 21, 31, 41, 51, 61) transmits branch network information including at least identification information on a branch network to which it belongs at a predetermined timing, and based on branch network information received from an adjacent hub, determines whether or not to add the network type information to a signal to be transmitted to the adjacent hub.
6. A network setting method in a train network system
that includes a plurality of hubs (11, 21, 31, 41, 51, 61)
installed in train vehicles, each of the plurality of hubs
(11, 21, 31, 41, 51, 61) belonging to both a trunk network
in which a signal transmission range is not limited and a
branch network in which a signal transmission range is
limited, and adding, to a signal to be transmitted to a hub
belonging to the same branch network as it does, network
type information indicating whether the signal is a trunk

network signal or a branch network signal, the network setting method comprising:
an information transmission step, by a hub having joined a branch network, of transmitting branch network information including information necessary for setting to newly join the branch network to which the hub have joined; and
a setting step, by a hub not yet having joined a branch network, of determining a branch network to join and performing setting of the branch network, based on the branch network information transmitted in the information transmission step.