Although it is the button node, it is more to do with the message coming out of it.
This is the node code:
[
    {
        "id": "89af2b10f87e66e9",
        "type": "ui_button",
        "z": "f6a2d75e7dd8f7ed",
        "name": "",
        "group": "66644b9721f1197e",
        "order": 0,
        "width": 0,
        "height": 0,
        "passthru": false,
        "label": "Start",
        "tooltip": "",
        "color": "",
        "bgcolor": "",
        "className": "",
        "icon": "",
        "payload": "TRIP",
        "payloadType": "str",
        "topic": "TRAIN",
        "topicType": "msg",
        "x": 350,
        "y": 370,
        "wires": [
            [
                "0f5725cd2cbfc233",
                "a5d2da0031c49502"
            ]
        ]
    },
    {
        "id": "66644b9721f1197e",
        "type": "ui_group",
        "name": "Control",
        "tab": "530b9cb957853dd9",
        "order": 1,
        "disp": true,
        "width": "3",
        "collapse": false,
        "className": ""
    },
    {
        "id": "530b9cb957853dd9",
        "type": "ui_tab",
        "name": "Track Singal NEW",
        "icon": "dashboard",
        "order": 43,
        "disabled": false,
        "hidden": false
    }
]
The topic field is set to TRAIN.
But this is what I see when I press it:
Where's the topic?


 
  