On a sectional chart, a Military Training Route (MTR) is depicted as:
Why →
MTRs are depicted as thin gray (or purple, depending on chart series) lines labeled with 'IR' (Instrument Route) or 'VR' (Visual Route) followed by a route number. Military aircraft may operate at high speeds along these routes, often below 10,000 feet. Route numbers with 3 digits (e.g., IR-123) generally indicate operations at or below 1,500 feet AGL.The trap →
MTR lines can be easy to miss visually because they do not bound an area. Pilots who only look for airspace boundaries overlook them.SOURCE → FAA Aeronautical Chart User's Guide; Advisory Circular 00-63CHECKED APR 21ACS II.A.K4EASY