Most of Amtrak's routes are likely to be on the NEC, where the top speed is I believe 140 mph (~200 km/h). Outside the NEC, the newest of the diesel fleet can probably reach 100mph (or 160 km/h), but the actual routes are unlikely to support that kind of speed with any amount of routine. Instead, the effective speed limit (again outside NEC) is going to be closer to ~60-70mph.