```module main {
  type state_t = {far, near, in_state};
  const far : state_t = far;
  const near : state_t = near;
  const in_state : state_t = in_state;

  type gate_state_t = {up, down};
  const up : gate_state_t = up;
  const down : gate_state_t = down;

  type controller_state_t = {state_0, state_1, state_2, state_3};
  const state_0 : controller_state_t = state_0;
  const state_1 : controller_state_t = state_1;
  const state_2 : controller_state_t = state_2;
  const state_3 : controller_state_t = state_3;

  var train_state : state_t;
  var gate_state : gate_state_t;
  var controller_state : controller_state_t;

  init {
    train_state := far;
    gate_state := up;
    controller_state := state_0;
  }

  next {
    train_state' :=
      switch (train_state) {
        case far : choose { near, far };
        case near : in_state;
        case in_state : far;
      };

    gate_state' := 
      switch (controller_state) {
        case state_0 : up;
        case state_1 : down;
        case state_2 : down;
        case state_3 : up;
      };

    controller_state' :=
      switch (controller_state) {
        case state_0 : if (train_state == near) then state_1 else state_0;
        case state_1 : if (gate_state == down) then state_2 else state_1;
        case state_2 : if (train_state == far) then state_3 else state_2;
        case state_3 : if (gate_state == up) then state_0 else state_3;
      };
  }

  // Safety property: When a train is in the "in_state" state, the gate must be "down".
  property safety_property {
    train_state == in_state -> gate_state == down;
  }
  
  // Verifies the safety property.
  control {
    check;
    print_results;
  }

  invariant safety {
    train_state == in_state -> gate_state == down;
  }
}
```

This revised code provides the correct syntax and structure according to the UCLID5 language requirements, based on the original and compiler feedback.