It could be an option. But if the rumoured dispute between KMB and Neoman has any substance, I don't think that would happen.
Going back to the ISBe will make more sense in that the integration should be a no-risk job, but it may take away the potential benefit that the E400/500 could share the parts by having the same engine type - modern engines like the ISLe can adjust their power with only a bit of software change in their engine management system. If they think the current E400(HK) has too much power, why not tuning the engine power down a bit instead? There is no guarantee that, at similar power output, the ISB would be more economical to run anyway...
fai: I beg to differ. Coaches have a completely different operational regime and need the extra power to work well on motorways, and a citybus doesn't in city traffic. At the end of the day, the E400 was brought to HK because it is supposed to be cheaper to run than the existing tri-axles. If it has too much power, it makes sense to sacrifice a bit of that to get better fuel economy. Thing is, at full load the E400 can only carry a bit over 18 tons, that's 75% of the legal limt of a tri-axle. If the E500s can live happily at 330/340bhp, I reckon a 280bhp would be more than enough for the E400(HK) to do its job. I'll put it this way - for the E500 to get the same power-to-weight ratio at full load as the current E400(HK), it would have to have a 427bhp(!) engine. Just think about that.
It's not for a bus of this type these days. As mentioned by another member, it's a weight restriction issue. Also bear in mind that buses nowadays are so much heavier even when unloaded - Despite the E400 is a "light-weight" double decker, the 10.1m version in the UK is still almost 2,000kg heavier than a Metrobus. That alone equates to the same loss of payload. The tri-axle Trident mitigated the issue on the 10.6m version with a 3rd-axle, giving more leeway on weight and sacrifice the usable low-floor space. For a bus of this size, it's just a trade-off.
The E400(HK), with a capacity of 88, is not doing badly actually. The UK ones can only take up to 91 people anyway, and that's without the additional weight of the air-cond.
