I have visited this triline twice, once in 2011 and once in 2018 as part of the IBRG BELEX trip. This report is from the 2018 visit. Details of my 2011 visit are here. Like BEDELU the borders at this triline (not tripoint) are complicated because of the condominium arrangements regarding the shared ownership of the Moselle between Germany and Luxembourg. Above the triline the river is shared, hence like BEDELU there are 3 points of interest.
DELUFRLU is the western technical tripoint and is where the condominium meets the border between Luxembourg and France in middle of the Moselle. DEDELUFR is the eastern technical tripoint where the borders of France and Germany meet on the side of the lock. In between these 2 points is the triline (not one single point) DEFRLU. This triline is approximately 114 m long and cuts across the island in the Moselle.
This is area is really interesting to visit. It is not easy to identify the particular points in the map above. DEDELUFR bring the most straightforward. It is possible to approximate the position of the triline and the whereabouts of DELUFRLU. In 2011 I managed breifly to access the island was not able to explore. In 2018, access to the lock side incurred the wrath of the French lock keeper.
Nevertheless, lots to see and explore, with the bridge between Luxembourg and Germany, the border between France and Germany with road and rail crossings and old border markers to discover. Being part of the IBRG visit in 2018 meant I found some markers that eluded me in 2011, but also I found that an old DE/FR marker had disappeared in the interim.
The French geo portal map below incorrectly draws the border between France and Luxembourg at the edge of the island whereas it ought as indicated on the Luxembourg map follow the Thalweg (middle of the river).
France – German side
On this side there is a lot to see, the lock is a focal point and views across to the island which the triline transects. Our first visit was to the bi national library where the iconic French monuments were a short distance away as was the road crossing from France to Germany. A short hop then to the lock side, finding border markers and the rail crossing border which was clearly demarcated.
French – German Border Markers
Railway Border Crossing
Perl (DE) and Apach (FR) stations are 1 km apart. Prior to 2008 there were no cross border services for 14 years since then services have been reestablished on the Thionville–Trier railway (Obermoselbahn) at weekends. There are plans to extend the service both in terms of frequency and reinstating a daily service by DB regional.
This is also a major freight route. Railway tracks as they cross borders are clearly demarcated with railway signs marking the beginning and end of their jurisdictions. There are normally lots of signs regarding danger too. This is my second railway border crossing, my first being between Austria and Germany at the Griesen – Ehrwald-Schanz Border Crossing details here.
Views from the bridge
Schengen – the Luxembourg side. Monuments galore.
Views of the triline from Luxembourg
YouTube Videos
Date of Visit: 27.04.2018
Barry's Borderpoints 
Barry, concerning the bridge your findings are not correct. You must distinguish maintenance/construction and legal situation. Construction/maintenance is been by one of the country. Thats why the bridge of Schengen shows a German country plate at the beginning and is declared as B(undesstrasse) 419, the national road along the Mosel in Germany up to Trier.
From the legal side you stay in a country until you reach the other river side. So coming p.ex. from Schengen you are entering Germany only, when you are reaching the Perl river side. Be aware, when you think now ” I have just to jump to the other river side, and the police couldn’t follow”, in fact they can. They have an agreement, at least between Germany and Luxembourg, they just give a ring to their colleagues and can follow the next 15 km / 9 miles. After this, the local police will then take relay.
But for the river you are right, this are shared by both countries, only pay attention that the laws are not the same, so the Germen Wasserschutzpolizei might punish you for things, where the Luxembourg Service de la navigation doesn’t.
Yes it’s tricky.
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