This is how you download a KML from alltrails.com without needing a PRO account.

_readme.md

How to download the KML from an AllTrails page without PRO account

1. Open a trail page in your favourite browser, for instance: https://www.alltrails.com/explore/trail/us/arizona/south-kaibab-to-phantom-ranch-to-bright-angel-trail
2. Open the developper console
3. copy and paste the script below (script.js)
4. execute it, you should see a KML file being downloaded

The last file below is for advanced users.

Enjoy!

script.js

(function () {
	const API_KEY = '3p0t5s6b5g4g0e8k3c1j3w7y5c3m4t8i';
	const URL = 'https://www.alltrails.com/api/alltrails/trails/';
	const DETAILS_LEVEL = 'medium';

	function decoded_objects_from_polyline(t) {
		var e = [5, 5];
	    length = t.length,
	    dimensions = e.length;
	    var i = 0
	      , n = 0
	      , a = 0
	      , r = 0;
	    for (buffer = new Array(dimensions * length,0),
	    bufferCount = 0; i < length; ) {
	        r = null;
	        for (var s = 0; s < dimensions; s++) {
	            a = 0,
	            n = 0;
	            do {
	                if (i >= length)
	                    break;
	                r = t.charCodeAt(i++) - 63,
	                a |= (31 & r) << n,
	                n += 5
	            } while (r >= 32);delta = 0 != (1 & a) ? ~(a >> 1) : a >> 1,
	            lastValue = 0 == bufferCount ? 0 : buffer[(bufferCount - 1) * dimensions + s],
	            buffer[bufferCount * dimensions + s] = Math.round((lastValue + Math.pow(10, -e[s]) * delta) * Math.pow(10, e[s])) / Math.pow(10, e[s])
	        }
	        bufferCount += 1
	    }
	    return buffer = buffer,
	    rows = bufferCount,
	    res = [],
	    buffer.each_slice(dimensions, function(t) {
	        res.push(t)
	    }),
	    res
	}

	function download(obj) {
		var a = window.document.createElement('a');
		a.href = window.URL.createObjectURL(new Blob([obj.text], {type: 'text/xml'}));
		a.download = obj.name;
		document.body.appendChild(a);
		a.click();
		document.body.removeChild(a);
	}

	function createKml(obj) {
		var kml = '<?xml version="1.0" encoding="UTF-8"?><kml xmlns="http://www.opengis.net/kml/2.2"><Document><name>' + obj.name + '</name><Placemark><name>' + obj.name + '</name><LineString><altitudeMode>relativeToGround</altitudeMode><coordinates>';
		kml += obj.pointsData.map(function (p) {
			return p.reverse().join() + ',0';
		}).join(' ');
		kml += '</coordinates></LineString></Placemark></Document></kml>';
		return {name:obj.name.replace(/[^\w]/ig,'')+'.kml', text:kml};
	}

	function processResponse(response) {
		var trail = response.trails[0];
		var pointsData = decoded_objects_from_polyline(trail.defaultMap.routes[0].lineSegments[0].polyline.pointsData);
		return {name:trail.name, url:document.location.href, pointsData:pointsData};
	}

	function getTrailsData(trailId) {
		return $.ajax({
			url: URL + trailId + '?detail=' + DETAILS_LEVEL,
			type: "GET",
			beforeSend: function (xhr) {
				xhr.setRequestHeader('x-at-key', API_KEY);
			}
		});
	}

	function getTrailId() {
		var dfd = jQuery.Deferred();
		var alternateEl = $('link[rel="alternate"]')[0];
		var data = $('li.favorite-menu').attr('data-react-props');
		if (alternateEl) {
			dfd.resolve($('link[rel="alternate"]')[0].href.split('/').pop());
		} else if (data) {
			dfd.resolve(JSON.parse(data).trailId);
		} else {
			dfd.reject('Could not identify the trail ID. Please enter a trail specific page.');
		}
		return dfd.promise();
	}

	function init() {
		getTrailId().then(getTrailsData, console.error)
					.then(processResponse)
					.then(createKml)
					.then(download);
	}

	Array.prototype.each_slice = function(t, e) {
		for (var i = 0, n = this.length; i < n; i += t) e.call(this, this.slice(i, i + t));
	}

	init();
	
})();

script_advanced.js

//Go to https://www.alltrails.com/trail/new and copy the response obtained from osmsegments

(function () {
	function decoded_objects_from_polyline(t) {
		var e = [5, 5];
		length = t.length,
		dimensions = e.length;
		var i = 0, n = 0, a = 0, r = 0;
		for (buffer = new Array(dimensions * length,0),
		bufferCount = 0; i < length; ) {
			r = null;
			for (var s = 0; s < dimensions; s++) {
				a = 0,
				n = 0;
				do {
					if (i >= length)
						break;
					r = t.charCodeAt(i++) - 63,
					a |= (31 & r) << n,
					n += 5
				} while (r >= 32);delta = 0 != (1 & a) ? ~(a >> 1) : a >> 1,
				lastValue = 0 == bufferCount ? 0 : buffer[(bufferCount - 1) * dimensions + s],
				buffer[bufferCount * dimensions + s] = Math.round((lastValue + Math.pow(10, -e[s]) * delta) * Math.pow(10, e[s])) / Math.pow(10, e[s])
			}
			bufferCount += 1
		}
		return buffer = buffer,
		rows = bufferCount,
		res = [],
		buffer.each_slice(dimensions, function(t) {
			res.push(t)
		}),
		res
	}

	function download(obj) {
		var a = window.document.createElement('a');
		a.href = window.URL.createObjectURL(new Blob([obj.text], {type: 'text/xml'}));
		a.download = obj.name;
		document.body.appendChild(a);
		a.click();
		document.body.removeChild(a);
	}

	function createKml(obj) {
		var kml = '<?xml version="1.0" encoding="UTF-8"?><kml xmlns="http://www.opengis.net/kml/2.2"><Document><name>' + obj.name + '</name>';
		obj.pointsDatas.forEach(function (pointsData, index) {
			kml += '<Placemark><name>' + obj.name + '_' + index + '</name><LineString><altitudeMode>relativeToGround</altitudeMode><coordinates>';
			kml += pointsData.map(function (p) {
				return p.reverse().join() + ',0';
			}).join(' ');
			kml += '</coordinates></LineString></Placemark>';
		});
		kml += '</Document></kml>';
		return {name:obj.name.replace(/[^\w]/ig,'')+'.kml', text:kml};
	}

	function processLineSegments(lineSegments) {
		var pointsDatas = [];
		lineSegments.forEach(function (lineSegment) {
			pointsDatas.push(decoded_objects_from_polyline(lineSegment.polyline.pointsData));
		});
		download(createKml({
			name: 'trail',
			url: document.location.href,
			pointsDatas: pointsDatas
		}));
	}

	function init() {
		processLineSegments([paste lineSegments here]);
	}

	Array.prototype.each_slice = function(t, e) {
		for (var i = 0, n = this.length; i < n; i += t) e.call(this, this.slice(i, i + t));
	}

	init();
	
})();