Revolute/Continuous joints not loading in Gazebo [closed]

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I am making a cargo cart to attach to a Clearpath Robotics Ridgeback robot but my revolute and continuous joints don't seem to load in Gazebo. The colors defined for links also don't load in Gazebo.

I am attaching the code that I have added to the Ridgeback urdf file in order to incorporate the cart. Please take a look and help me with the issue.

<joint name="tow_hitch_joint" type="fixed">
  <origin xyz="-0.319 0 0.280" rpy="0 0 0" />
  <parent link="top_link"/>
  <child link="tow_hitch_link" />
</joint>

<link name="tow_hitch_link">
  <visual>
    <origin xyz="0 0 0.0125" rpy="0 0 0" />
    <geometry>
      <cylinder radius="0.05" length="0.025"/>
    </geometry>
    <material name="yellow"/>
  </visual>
  <collision>
    <origin xyz="0 0 0.0125"/>
    <geometry>
      <cylinder radius="0.05" length="0.025"/>
    </geometry>
  </collision>
</link>

<joint name="cargo_cart_joint" type="revolute">
  <origin xyz="0 0 0.025" rpy="0 0 0" />
  <parent link="tow_hitch_link"/>
  <child link="cargo_cart_link" />
  <axis xyz="0 0 1" />
  <limit effort="0" lower="-0.785" upper="0.785" velocity="0" />
</joint>

<link name="cargo_cart_link">
  <visual>
    <origin xyz="-0.2 0 0.0125" rpy="0 0 0" />
    <geometry>
      <box size="0.5 0.643 0.025"/>
      <material name="black"/>
    </geometry>
  </visual>
  <collision>
    <origin xyz="-0.2 0 0.0125"/>
    <geometry>
      <box size="0.5 0.643 0.025"/>
    </geometry>
  </collision>
  <inertial>
    <origin xyz="0 0 0" rpy="0 0 0"/>
    <mass value="0.05"/>
    <inertia
      ixx="4.4744" ixy="0.03098" ixz="0.003647"
      iyy="7.1624" iyz="0.1228"
      izz="4.6155"/>
  </inertial>
</link>