Yes - that is exactly why you cannot see it - the route / trip is lying exactly on top of it.
One of the advantages of having the track as well as the route or trip - is that if the route / magenta line is recalculated by the XT, the black track becomes visible. Setting the width of the track doesn't help - because it also changes the width of the magenta line.

- Track and Route.png (120.35 KiB) Viewed 1284 times
In the screen shot below the motorcycle, you can see the black track underneath the magenta route. The magenta has a black outline.
Where the track and the route separate, the magenta line goes ahead. The black track goes off to the left.
If you want this black border effect then you have to edit your theme. Themes are used by the XT to allow you some choice as to how your map is drawn on the screen.
If you want to plan your own theme - it is very easy to modify an existing one. There is a whole list available from the XT screen under Settings -> Map & Vehicle -> Map Theme. Find one that you like. The rest of this post is to do with taking a copy of an existing theme and modifying it slightly.
It is actually very simple to do. Change a couple for words and a couple of numbers. I've just written a step by step guide.
As
@Peobody pointed out - it isn't necessary. Seeing the black outline just gives that reassurance that the track is actually being displayed on the screen. But you get that reassurnace anyway if you look at the screen before loading the route - you will see the entire track.
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Modifying a Copy of a Theme file
To modify a theme you need to be able to access your XT using a USB cable and a computer. ie plug the cable into the XT and accept the message that is displayed on the screen to enter File Transfer Mode.
The computer should eventually repsond and show the Zumo XT as a device in a File Explorer. On a Windows machine, Zumo XT is listed as one of the entries under 'This PC'. In there is an entry called 'Internal Storage'. There may be another entry called 'Memory Card' if you have an SD card inserted in the XT. Select Internal Storage.
This shows a whole list of folders - one of which is called '
Themes'.

- themes.png (48.04 KiB) Viewed 1284 times
If you select that, and then select 'Map', then that shows a list of all of the themes that are available to the XT. These all have the extension
.kmtf . You can choose the theme that you wish to use from the XT screen. The file names are recognisable from the theme name - but they are not identical.

- ThemeList.png (55.49 KiB) Viewed 1284 times
The above screen shot shows my list of themes. The ones beginning with JFH are ones that I have modified from existing themes.
To do this, find a them that you like and copy the original theme file to your computer. Immediately name it as something else.
So for example - I took United Kingdom.kmtf copied it to my disk drive and immediately renamed it
JFH-UK-XT-2.kmtf
I then made another copy and called renamed that copy
JFH-UK-XT-3.kmtf
Both files at this point are identical to the original. So I then loaded a plain text editor (Notepad on my Windows computer) and set about changing the contents. This is quite easy.
At the top of the file is a section which looks like this
<GARMIN_MAP_THEME>
<METADATA>
<TITLE>United Kingdom</TITLE>
<DESCRIPTION>Theme for United Kingdom.</DESCRIPTION>
<LOCALELIST>
<locale>United Kingdom</locale>
</LOCALELIST>
<Version><Major>1</Major><Minor>00</Minor></Version>
</METADATA>
I altered this slightly:
<GARMIN_MAP_THEME>
<METADATA>
<TITLE>JFH-UK-XT-2</TITLE>
<DESCRIPTION>JFH-UK-XT-2</DESCRIPTION>
<LOCALELIST>
<locale>United Kingdom</locale>
</LOCALELIST>
<Version><Major>1</Major><Minor>00</Minor></Version>
</METADATA>
Two changes - that is all. What you place here is what the XT displays when you are choosing the theme.
-------------------
Near to the bottom of the file is a section that looks like this
<STYLE field = "MAP_TRK_CLR" scale = "1.00" >
<COLOR >
<PRIMARY day = "#06EEEE" night = "#06EEEE" />
<SECONDARY day = "#0A92E2" night = "#0A92E2" />
</COLOR>
</STYLE>
<STYLE field = "MAP_TRK_SAVED_CLR" scale = "1.00" border = "1.00" >
<COLOR >
<PRIMARY day = "#06EEEE" night = "#06EEEE" />
<SECONDARY day = "#0A92E2" night = "#0A92E2" />
</COLOR>
</STYLE>
I changed just two values from 1.00 to 2.00
<STYLE field = "MAP_TRK_CLR" scale = "2.00" >
<COLOR >
<PRIMARY day = "#06EEEE" night = "#06EEEE" />
<SECONDARY day = "#0A92E2" night = "#0A92E2" />
</COLOR>
</STYLE>
<STYLE field = "MAP_TRK_SAVED_CLR" scale = "2.00" border = "1.00" >
<COLOR >
<PRIMARY day = "#06EEEE" night = "#06EEEE" />
<SECONDARY day = "#0A92E2" night = "#0A92E2" />
</COLOR>
</STYLE>
Then resave the kmtf file - which if you remember, you have already called something else. The name that I used ie JFH-UK-XT-2.kmtf represents the number 2 that I used in the scale in that last section.
I repeated the exercise with the other file JFH-UK-XT-3.kmtf except, I used a scale of 3.00 instead of 2.00. To see what the difference was. That way it was easy to experiment to find out which I liked best, just using the XT screen.
All you have to do then is to copy your new kmtf files into the Internal Storage/Themes/Map folder. You haven't changed the original files - you have just created one or more new ones to experiment with.
So when you restart your XT, head to
Settings -> Map & Vehicle -> Map Theme If your new ones don't appear, just wait for the list to populate.
Note that the width that you set for the track using the XT screen is then multiplied by the new scale factor. I find that keeping that to medium is the best way.
When zooming in and zooming out, the scale factor seems to be a little slow in responding - but it rapidly settles down. Some times it is hard to see - but mostly the black outline shows up clearly around the edge of the magenta route.