解:$(2)$由题意,得客车经过$600\div 60 = 10(\mathrm {h})$到达目的地,
出租车经过$600\div 100 = 6(\mathrm {h})$到达目的地。
$ $当$t = 2$时,两车相距$600 - 2×(60 + 100)=280(\mathrm {km});$
$ $当$t = 5$时,两车相距$5×(60 + 100)-600 = 200(\mathrm {km});$
$ $当$t = 8$时,出租车已经到达目的地,此时两车相距$8×60 = 480(\mathrm {km})。$
$ (3)$设$A$加油站到甲地的距离为$x\mathrm {km}。$
$ $当$A$加油站比$B$加油站更靠近甲地时,根据题意,得$\frac {x}{60}=\frac {600 - 200 - x}{100},$
$ 100x = 60×(400 - x)$
$ 100x = 24000 - 60x$
$ 100x + 60x = 24000$
$ 160x = 24000$
$ x = 150;$
$ $当$A$加油站比$B$加油站更靠近乙地时,根据题意,得$\frac {x}{60}=\frac {600 - x + 200}{100},$
$ 100x = 60×(800 - x)$
$ 100x = 48000 - 60x$
$ 100x + 60x = 48000$
$ 160x = 48000$
$ x = 300。$
答:$A$加油站到甲地的距离为$150\ \mathrm {km}{或3}00\ \mathrm {km}。$