Well you may think it won't work well and I'm not a train driver, but I do have 40 plus years of hands on, down in the dirt, road building in about every capacity from ditch digger to superintendent and laying hundreds of miles of pipe, from 4" drainage on up to 8' box culverts, most always under the watchful eye of NYS DOT engineers and inspectors. Although there are some places that require uncompacted bedding, we were required to spread the specified select fill in the trench, grade it to 1/4 " tolerance, tamp it to specs and set the pipe. If jointed pipe like concrete was being set, a slot for the bell would be shoveled out as it was set. Then the select backfill would be uniformly spread in the trench on both sides in apprx 4" lifts and leveled by laborers who were REQUIRED to poke the fill under the pipe and tamp it in at an angle with either a shovel handle or 2x4 etc., to fill slight voids, corrugations and compact the material . Not perfect but better than no compaction. The angled hand tamping was necessary because the side of the vibratory tamper will hit the pipe and leave 6" or more of uncompacted material on all pipe 12"or larger. As the simplified illustration below demonstrates, most of the compaction zone force is directly under the drum/plate/etc. and drops off rapidly to the side ---- A plate tamper, trench roller, etc. would then make the specified number of passes to obtain the correct compaction that is determined by the backfill material, moisture content and other specs. The following lifts are added following the same procedure up to half the diameter of the pipe, then the compactor will work right up to the pipe with no hand tamping necessary.
There are other ways to do it including that mentioned by /pine using more expensive but time saving materials like pea gravel or small crushed stone, because they are basically self compacting and the vibratory tamper just settles it into any voids. We used that method almost exclusively on highway shoulder underdrains, using a trencher that pulled a trailer carrying and laying 250 ft rolls of pipe and could do miles per day. I recall one 4 lane job over 20 miles long we did, where we put in nearly 100 miles that way.
Smiley