Possible Clues:

• International base where engines are checked
• Trials installation
• It's hard to do without lying in framework of trial
• Place for looking at engines, say, under trying conditions?
• River bottom as place for experimental trials
• Proving ground
• Hants runner's base in which working parts are put through their paces
• Experimental rig
• Where engines take a trial run
• Trial-run proving ground