-J param is not working for robots with more than 6 DOF

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Hello,

I have a robot that can be described as a robotic arm of 6 DOF attached to a mobile base (train) with 2 DOF. When I try to spawn the robot and set the initial value of the joints doing:

<!-- Spawn a robot into Gazebo -->
<node name="arm_train_spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-urdf -param robot_description -model schunk_lwa4p_DH_arm_and_train -J Joint_Train_Rail 10" respawn="false" output="screen" />

I have noticed that the train is only spawned at the position 10 of his prismatic joint if the robotic arm has 4 DOF ( 6 DOF of the whole model: 2 from the base + 4 from the arm). I would like to know if there is a way to split this xacro in 2 individual models, invoking them as 2 different robots, and attach one to another ?

Thank you in advance.

JLuis Samper