(check-expect (proper-train? (cons (make-unit 'L 2) (cons (make-unit 'C 3) empty))) true)
(check-expect (proper-train? (cons (make-unit 'L 4) (cons (make-unit 'L 2) (cons (make-unit 'C 3) empty)))) true)
(check-expect (proper-train? (cons (make-unit 'B 4) (cons (make-unit 'L 2) (cons (make-unit 'C 3) empty)))) false)
(check-expect (proper-train? (cons (make-unit 'B 4) (cons (make-unit 'T 2) (cons (make-unit 'P 3) empty)))) true)
(check-expect (proper-train? (cons (make-unit 'C 3) (cons (make-unit 'C 4) empty))) true)
(check-expect (proper-train? (cons (make-unit 'C 3) (cons (make-unit 'L 4) empty))) false)