You will find the route relations in the
planet_osm_line table, with a negative osm_id field value corresponding to the OSM relation ID.
Once you have loaded the table, you can set a filter on the table with rightclick on the layer entry:
"osm_id" = '-51855'
to get only the trail you want, and then export the result to the format you want.
If you want a multiline out of it, perform
Vector -> Geometry Tools -> Single part to Multipart. The only thing that QGIS can not provide (yet) is a single continuos line out of the OSM data.