How do I track the position of a moving svg element in relationship to a fixed svg element?

Question

Try placing your path in an svg 'wrapper/empty' element. Then use getBBox() for the wrapper. This will provide x,y values used to set/read the path's translations.

EDIT: I have included an example below that can move rects between reference lines. This works even when the rect has been previously transformed. This uses a single SVG Wrapper. Note: If you are only going to transform your element a single time, then you do not have to use the matrix transform on the bbx, bby values. they can be used directly.

<!DOCTYPE html>
    <html xmlns="http://www.w3.org/1999/xhtml">
        <head>
        <title>SVG Wrapper</title>
        <meta http-equiv="content-type" content="text/html; charset=UTF-8">
        </head>
        <body style='font-family:arial'>
            <center>
            <h4>SVG Wrapper</h4>
            <div style='width:90%;background-color:gainsboro;text-align:justify;padding:10px;border-radius:6px;'>
           Translate rects to line via SVG Wrapper. Can translate between lines
            </div>
            <div id="svgDiv" style='background-color:gainsboro;width:400px;height:400px;'>
<svg id="mySVG" width="400" height="400">
<line id=refLine1 stroke='blue' stroke-width='2' x1=20 y1=40 x2=380 y2=40 />
<line id=refLine2 stroke='maroon' stroke-width='2' x1=20 y1=100 x2=380 y2=100 />
<g id=rectG>
<rect id=redRect fill=red x=60 y=200 width=100 height=50 />
<rect id=greenRect fill=lime x=260 y=320 width=100 height=50 />
</g>
<svg id=Wrapper />
</svg>
            </div>
<center>
<button onClick=moveToLine(refLine1,redRect)>Move Red Rect to <span style=color:blue>Line 1</span></button>
<button onClick=moveToLine(refLine1,greenRect)>Move Green Rect to <span style=color:blue>Line 1</span></button>
<br />
<button onClick=moveToLine(refLine2,redRect)>Move Red Rect to <span style=color:maroon>Line 2</span></button>
<button onClick=moveToLine(refLine2,greenRect)>Move Green Rect to <span style=color:maroon>Line 2</span></button>
</center>
            <br />SVG Source:<br />
            <textarea id=svgSourceValue style='font-size:110%;font-family:lucida console;width:90%;height:200px'></textarea>
            <br />Javascript:<br />
            <textarea id=jsValue style='border-radius:26px;font-size:110%;font-weight:bold;color:midnightblue;padding:16px;background-color:beige;border-width:0px;font-size:100%;font-family:lucida console;width:90%;height:400px'></textarea>
            </center>
            <div id='browserDiv' style='padding:5px;position:absolute;top:5px;left:5px;background-color:gainsboro;'>OK in:IE11/CH32/FF28<br /></div>
<script id=myScript>
function moveToLine(refLine,rect)
{
    //---move rect to this location at line---
    var targetY=refLine.y1.baseVal.value
    var targetX=rect.x.baseVal.value
    Wrapper.appendChild(rect)
    var bb=Wrapper.getBBox()
    //---return rect to its <g>---
    rectG.appendChild(rect)
    var bbx=bb.x
    var bby=bb.y

    //---bind wrapper x,y to rect's current transform--
    var pnt = mySVG.createSVGPoint();
    pnt.x = bbx;
    pnt.y = bby;
    var sCTM = rect.getCTM();
    PNT = pnt.matrixTransform(sCTM.inverse());
    //---translate rect's x,y to target---
    var transX=targetX-PNT.x
    var transY=targetY-PNT.y

    rect.setAttribute("transform","translate("+transX+" "+transY+")")

    svgSourceValue.value=svgDiv.innerHTML
}

</script>
            <script>
            document.addEventListener("onload",init(),false)
            function init()
            {
                svgSourceValue.value=svgDiv.innerHTML
                jsValue.value=myScript.text
            }
            </script>
        </body>
    </html>